Thank you to Imogen. Definitions are underlined in the clues.Across
1. Rubbish clothes, baggier with holes at intervals (7)
GARBAGE : GARB(clothes/attire) + “baggier” minus its 1st, 3rd and 5th letters(with holes at intervals).
5. Wasted help, say, to get in trim (7)
SHAPELY : Anagram of(Wasted/drunk) HELP, SAY.
10. Hungry feeling? Frier finally working (4)
PANG : PAN(an implement for frying food/a frier) + last letter of(finally) “working”.
11. Exercising, animate toes almost in sequence (3,2,1,4)
ONE AT A TIME : Anagram of(Exercising) [ANIMATE + “toes” minus its last letter(almost) ].
12. Cup from Scotland, or one from Hobart? (6)
TASSIE : Double defn: 1st: An old Scottish word for a small drinking cup; and 2nd: An informal Australian term for one from Hobart, capital of Tasmania, Australia.
13. Called out indecent state of the crew (8)
NAUTICAL : Homophone of(Called out) “naughty”(improper/indecent) + CAL(abbrev. for the US state of California).
Defn: Of/describing the crew of a seacraft.
14. New magazine from US island (9)
STATESMAN : STATES(or the States, an informal term for the USA) + MAN(or Isle of Man, an island in the Irish Sea).

16. Looking west, this elevated point used to measure height and circumference (5)
GIRTH : Reversal of(Looking west, in an across clue) TRIG(or trig point/a fixed point at a high elevation used to survey and measure distances on the ground, calculated using trigonometry) + H(abbrev. for “height”).
17. Peach burst open (5)
SPLIT : Double defn: 1st: Describing a peach whose pit has cracked/split open Like “peach”, slang for “to inform on/betray one”; and 2nd: Burst open/to divide forcefully.
19. Beautiful girl whose twin perished (4,5)
SNOW WHITE : Anagram of(… perished) WHOSE TWIN.
Defn: The fairy tale …, the fairest one of all.
23. Pope’s half genial proclamation (8)
BENEDICT : 1st 3 letters of(half) “benign”(pleasantly warm/genial) + EDICT(a decree/a formal declaration requiring obedience).
Defn: A former … of the Catholic Church.
24. The precise hour business is over, close up (6)
O’CLOCK : Reversal of(… is over) CO(abbrev. for “company”, a business) + LOCK(to close up/to secure).
Defn: Derived from “of the clock” and used after a number from 1 to 12 to tell the exact/precise hour of the day.
26. A couple of hairstyles, extremely smart feature of the Arctic (10)
PERMAFROST : [ PERM(short for “permanent wave”, a hairstyle with curls from chemical treatment) + AFRO(a hairstyle where curly hair is combed out into a bushy rounded shape) ](a couple of hairstyles) + 1st and last letters of(extremely) “smart”.
Defn: …, viz. ground that is completely frozen at or below zero degrees C for at least 2 consecutive years.
27. In rage, black a source of energy (4)
BOIL : B(abbrev. for “black”) + OIL(petroleum, a source of energy).
Defn: …/seething with anger.
28. Lousy payment for cartoon (7)
PEANUTS : Double defn: 1st: A trivial sum in wages/a pittance; and 2nd: The cartoon series/comic strip by Charles M.Schulz.
29. Hear about one record in a set of three (7)
TRILOGY : TRY(to hear a case in a court of law) containing(about) [ I(Roman numeral for “one”) + LOG(a chronological record of, say, events) ].
Defn: … closely related creative works.
Down
2. Unyielding squad a man triumphantly arrests (7)
ADAMANT : Hidden in(… arrests) “squad a man triumphantly”.
3. Fraudulent attempt to climb into conveyance (5)
BOGUS : Reversal of(… to climb) GO(an attempt/a try) contained in(into) BUS(a form of transport/conveyance).
4. Creators of digital protection are not just good friends? (7)
GLOVERS : G(abbrev. for “good”) LOVERS(those who are more than/not just friends or good friends).
Defn: …, viz. protection for your digits/fingers.
6. In base, extravagant to luxuriate in this? (3,3)
HOT TUB : HUB(a central or focal point of, say, activities/a base) containing(in …, …) OTT(abbrev. for “over the top”/unrestrained/extravagant).
“This” in the clue:

7. Spirited, transforming hotel into a place of culture (5,4)
PETRI DISH : Anagram of(…, transforming) SPIRITED + H(letter represented by “hotel” in the phonetic alphabet).
Defn: …, that is, the culture/cultivation of living material in the laboratory.
8. Roast meat: like a bit of turkey? (7)
LAMBAST : LAMB(meat derived from young sheep) + AS(like/similar to in comparison, as in “hard as nails”) + 1st letter of(a bit of) “turkey”.
Defn: …/to harshly criticise someone.
9. Being born again, robe outwardly in pink (13)
REINCARNATION : 1st and last letters of(… outwardly) “robe” + IN + CARNATION(a pink colour).
Defn: A being that has been born again.
15. Fellow in community had injured brainstem (9)
TRIBESMAN : Anagram of(injured) BRAINSTEM.
Defn: …/a man belonging to a tribe.
18. Arrive before opening of park, and go back (7)
PRECEDE : 1st letter of(opening of) “park” plus(and) RECEDE(to go back/withdraw).
20. Bertie develops from bad to worse (7)
WOOSTER : Anagram of(bad) TO WORSE.
Answer: Bertie … aka Bertram Wilberforce, the fictional character in the Jeeves series of books by P.G. Wodehouse.
21. About to go off perhaps, indicating right (7)
TICKING : Double defn: 1st: Describing, say/perhaps, a time bomb; and 2nd: Making a check mark against something to indicate that it is correct.
22. Weirdo following one in poor visibility (6)
MISFIT : [ F(abbrev. for “following”) + I(Roman numeral for “one”) ] contained in(in) MIST(one possible cause of poor visibility/very much reduced distance within which thangs can be clearly seen, literally or figuratively).
25. Brief description of timeless ballet dancing (5)
LABEL : Anagram of (… dancing) “ballet” minus(…-less) “t”(abbrev. for “time”).
Defn: …/a word or short phrase assigned to a person or group to classify their behaviour.
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I had to use the thesaurus for peach. It has another definition meaning to betray or become an informer, thus SPLIT.
Thank you for the blog, scchua, it clarified a lot of semi-parsed frustrations – especially with PANG (a wild guess) and GIRTH. (I knew of trigonometry, but didn’t know trig was a word.)
I also had never heard a peach called a SPLIT before.
Did anyone else start off trying TURNING for 21D?
PEANUTS and GLOVERS made me grin
I thought PERMAFROST was the pick of a great bunch, although SPLIT didn’t make much sense. Also liked GARBAGE, TRIBESMAN and TICKING. ADAMANT and REINCARNATION reminded me of recent puzzles with pink being the operative word.
Ta Imogen & scchua.
ADAMANT twice in a week. [Further to Bodycheetah’s comment on Monday, I once did the Prince Charming dance in front of my 450-strong year group during a punky Cinderella reimagining. Luckily, it was well received, despite lacking Diana Dors and a black panther.]
I struggled in the south west a little today but eventually prevailed.
No man is an island except the one that keeps appearing in puzzles. For once, I thought of it and solved STATESMAN quickly.
Likes: TRIBESMAN, O’CLOCK, NAUTICAL, GIRTH.
See my comment @1 on SPLIT, there is a rationale.
Thanks Imogen and scchua.
I thought that there were some very good clues here, PERMAFROST and PETRI DISH amongst others. Also baffled by SPLIT. Thanks to Imogen and to scchua for the excellent blog.
Enjoyable tussle but I still don’t really understand SPLIT.
NHO The Scottish cup but it had to be TASSIE.
Many thanks, both.
That definition of peach Martin @1 was new to me. Peanuts loi,a real penny-drop moment. Lots to like , thanks Imogen and scchua
Apologies, Martin, failed to take in your comment @1. Well spotted, that does it for me.
New for me: TASSIE = cup (but being from Australia, the other part of the clue was easy for me).
I could not parse SPLIT = peach (17ac).
Favourite: TICKING, PETRI DISH.
Wellbeck@2 – I started off with entering TURNING for 21d 😉
Did anyone else have TURNING for 21 DN, as in milk about to go off, and turning right? Held me up a bit.
I remembered “peach” from Oliver Twist, where Fagin is always afraid one of the pickpockets will inform on him. PERMAFROST was my favourite, too.
My first pass at 5ac led to BROCADE (trim?): homophones (“say”) of “broke” (wasted?) + “aid” (help).
Thanks Martin re PEACH (or rather SPLIT – I saw that particular meaning for PEACH, but didn’t see it for SPLIT). Blog corrected.
Trovatore @12: I tried that at first as well. Thanks Martin @1 for cracking the peach.
TonyM @10 – I fell into the same trap…thinking about it, RIGHT would have been RIGHT?, or RIGHT, SAY, for TURNING to have been correct.
Apart from that mucho enjoyable, just the parsing of GIRTH eluded me.
Thanks both.
Was chatting to a bloke in a bar. Told me he used to be in a famous band. I didn’t believe him but he was adamant.
Better than I usually do on a Imogen puzzle, who I find second only to Enigmatist for difficulty even if I couldn’t get the not heard of TASSIE.
Liked PERMAFROST, O’CLOCK, LAMBAST and WOOSTER amongst others.
Thanks schuaa and Imogen
WOOSTER my number one. Very smooth, and PEANUTS. Thanks both.
PEACH = grass is an old pre 1900 word, I think. Conan Doyle uses it in the Sherlock Holmes story, The Mazarin Stone…”if Icky Sanders has peached, I’ll swing for him…” or words to that effect.
So that clue was no problem to parse for me.
Lots of helpful anagrams greatly helped the cause today. It’s not often that both elements in a double meaning are unknown to me, but that was the reason that TASSIE took a while to find its way into the grid. And sadly, when I see the word Peach, I’m reminded of the very sad incident of the death of Blair Peach all those years ago, rather than another meaning for “inform on”…
Thanks, scchua. GIRTH my last in, and I couldn’t parse it. I didn’t know TASSIE was a Scottish word, as I know it from Sean O’Casey’s play The Silver Tassie, about a Dubliner in the First World War.
Thanks for the blog , good set of clues , GLOVERS , PETRI DISH , PEANUTS all very neat . TASSIE Tim from here gave me a prompt , I know where Hobart is .
PEACH , agree with Martin@1 and HYD@19 , slang for split , grass , snitch etc .
Like Wellbeck@2 and others , I had TURNING in mind but when I came to put the Downs in the C was there so had a rethink .
Does anyone visit the site Elevensquared ? Similar to this but more downmarket and lower quality although the coloured squares next to names are better . They use two colours for alternating horizontal bands , looks like Cornishware .