Thank you to Mudd. Definitions are underlined in the clues.
Across
1. Man left in class (6)
CASTLE : L(abbrev. for “left”) contained in(in) CASTE(one of the strata in a society where people are divided into different social classes).
Defn: A …/one of chess pieces.
4. Rapidly grow reputation finally in broadcast when leading party (8)
SNOWBALL : Last letter of(… finally) “reputation” contained in(in) SOW(to broadcast/spread) placed before(when leading) BALL(a party/social function that includes dancing).
9. Boring in empty office in recent years (2,4)
OF LATE : FLAT(boring/without energy) contained in(in) inner letters deleted from(empty) “office”.
10. Knitter’s heart invested in needlework, note (8)
CROTCHET : Middle letter of(…’s heart) “Knitter” contained in(invested in) CROCHET(a type of needlework).
Defn: A … in music.

12. Too much put on the counter in dollars — for a cheeky couple? (8)
BUTTOCKS : Reversal of(… put on the counter) OTT(abbrev. for “over the top”/too much) contained in(in) BUCKS(American term for dollars).
Defn: A couple of cheeks on one’s rear.
13. Little bit disturbed by new music genre (6)
TRANCE : TRACE(a little bit/a hint) containing(disturbed by) N(abbrev. for “new”).
Defn: An electronic dance ….
15. Bar in Canberra I like (4)
RAIL : Hidden in(in) “Canberra I like”.
16. Frozen assets of writer for the auditor? (10)
STATIONARY : Homophone of(… for the auditor) “stationery”(materials/implements useful for writing, which one might term a writer’s assets).
Defn: …/immobile.
19. He’s deeply sorry about a dozy type (10)
SLEEPYHEAD : Anagram of(… sorry) HE’S DEEPLY containing(about) A.
20. A stake in back (4)
ABET : A + BET(a wager/stake).
Defn: …/support.
23. Cross-Channel journey, might you say? Really go for it! (3,3)
LET RIP : LE TRIP(across the English Channel, in France, how one might term/might you say “the trip” in Franglais).
Defn: …/act with full force and without restraint.
25. Wine came with lust, worryingly (8)
MUSCATEL : Anagram of(…, worryingly) [ CAME plus(with) LUST ].
Defn: A sweet ….
27. Fellow, advanced in years, running (8)
MANAGING : MAN(fellow/chap) + AGING(advancing in years/advanced? in years).
Defn: …, say, an organisation.
28. Bizarre portal, perhaps? (3,3)
WAY OUT : Double defn: 2nd: An example/perhaps of which is a portal, a gateway.
29. On no account sign for legal officials (8)
NOTARIES : NOT(on no account/not ever) + ARIES(a sign of the zodiac).
30. My sale, awfully awfully small (6)
MEASLY : Anagram of(…, awfully) MY SALE.
Defn: …/a disapproving term for something that is way too small/little.
Down
1. Sock, clothing (7)
CLOBBER : Double defn: 1st: To hit hard; and 2nd: An informal term for a person’s ….
2. Patience: lose it when upset about broadcast (9)
SOLITAIRE : Anagram of(… when upset) LOSE IT containing(about) AIR(to broadcast, say, over the radio or television).
Answer: A game of solitary patience:

3. Cream smeared on it, looking for topping? (6)
LOTION : Anagram of(smeared) ON IT placed below(… topping, in a down clue) LO(look/see/looking for?).
5. Par for the course in Troon or Muirfield (4)
NORM : Hidden in(in) “Troon or Muirfield”.
And Troon and Muirfield are 2 Scottish golf courses, where scores are expressed as “so many strokes below or above par”.
6. Vital thing, where to go for battle (8)
WATERLOO : WATER(a vital thing/substance for living things) + LOO(slang for a toilet where one can go/urinate).
7. Pasty Shane chewed up (5)
ASHEN : Anagram of(… chewed up) SHANE.
Defn: …/pale.
8. Tory let off, almost no chance of winning? (7)
LOTTERY : Anagram of(… off) TORY LET.
Defn: A game of chance where the odds of winning are very slim/almost none.
11. Vague imprint in blue (7)
SKETCHY : ETCH(to fix/mark in one’s memory/imprint) contained in(in) SKY(the blue, a literary term).
14. Level in corporation, above which skills on the rise (7)
STRATUM : TUM(short for “tummy”/one’s stomach/corporation) placed below(above which …, in a down clue) reversal of(… on the rise, in a down clue) ARTS(acquired abilities in specific areas/skills).
Defn: … in society/a social class.
17. Too much to carry for bird (9)
ALBATROSS : Double defn: 1st: Metaphorically …/a heavy burden that hinders one’s progress – from the poem The Rime of the Ancient Mariner; and 2nd:

18. Canine, one attached to elastic? (8)
SPRINGER : Cryptically/?, something which can bounce/spring back on account of being attached to an elastic/a piece of material that can stretch and then return to its original shape.
Answer: Short for a “springer spaniel”, a hunting dog.

19. Lonely day for old ruler (7)
SOLOMON : SOLO(on one’s own/lonely) + MON(abbrev. for “Monday”).
Defn: …, in the Bible.
21. Russian author opening up before play (7)
TOLSTOY : Reversal of(…up, in a down clue) SLOT(a long narrow opening in an object) plus(before) TOY(to play/trifle with).
22. Grind over surface — tight corner (6)
SCRAPE : Double defn: 1st: To rub a rough hard surface against another; and 2nd: A difficult situation to be in.
24. Rule that may also be drawn up? (5)
TENET : A palindrome(… that may also be drawn up, in a down clue).
26. Top of head shaved in the past (4)
ONCE : “bonce”(slang for a person’s head) minus its 1st letter(Top of … shaved).
I enjoyed this – just a handful being beyond my (modest) capabilities.
My faves were Notaries and Solitaire.
Thanks to Mudd and scchua.
LOTION
Maybe…
looking for=LO, not in the usual way, but as a short form of Look Out?
An enjoyable crossword from Mudd – lighter than usual on the double definitions but 1d’s was very good. 12 was another of this setter’s trademarks! Another outing for MEASLY this week!
LOTTERY, ALBATROSS and ONCE (aah, memories of Rainbow’s Bungle Bonce) were favourites.
Thanks to Mudd and Scchua for the illustrated blog.
Thanks for the blog , good set of neat clues , many of them very concise and I think it is a good puzzle for newer solvers .
LOTION I am not convinced , maybe should just have been – look for topping – with the “for” telling us where to put LO .
I enjoyed this, although I had the inexplicable Grunge (a musical genre I have heard of) for 13a – I agree Trance (which I’ve never heard of) is the correct answer. It was my LOI. I looked up Gurge and saw that it was a whirlpool, so I thought this might explain the answer though I couldn’t understand why😂😂
Excellent blog
I enjoyed this. The only way I could parse LOTION was by the fact that I had TION (ON IT) looking for something to go on top of it – a cream, presumably – and perhaps indicated by the question mark. That was enough for me. That I then had an L from CASTLE . . .
Thanks Mudd for a clever set of clues with my top picks being BUTTOCKS, WAY OUT, NOTARIES, ALBATROSS, SOLOMON, and TOLSTOY. This was smooth sailing overall expect for failing with LOTION. Thanks scchua for the blog.
I agree with the points above, especially that it was on the easy end, clues were concise (a big tick from me), I enjoyed it, and I came to the same conclusion about LOTION as KVa.
I ticked RAIL, LOTTERY, ALBATROSS and MEASLY. I sort of liked DNK and LOI TRANCE.
Thanks Mudd and scchua
As Moly@5 with the music, although I had heard of Trance, so absolutely no excuse… agree LOOKING = LO is a bit off, maybe the -ing was a typo… not going to let that ruin a good fun crossword…
Thanks Mudd and Diane, nice albatross!
I used to enjoy TRANCE in its relatively distant heyday. It was still my last in. I found this straightforward but with some great clues. I was faster in the bottom half.
Liked BUTTOCKS, SLEEPYHEAD, MUSCATEL and WAY OUT.
Thanks Mudd and scchua.
Smooth puzzle. Enjoyed. Favourite was Tolstoy.
Just the right level of difficulty to get a few neurons firing. I liked 23a LET RIP. Thanks, @MUDD.