Thank you to Phi…is there a particular reason why he’s appearing on Sunday, which is not his usual spot…? Definitions are underlined in the clues.
… Yes there is. On this date (7 July) 100 years ago, the actor and entertainer named in the bottom row of the grid was born. Solutions include 3 highlights of his career.
Across
8 I try latest in ontology, backing philosophy student (4)
YOGI : Reversal of(…, backing) [I + GO(a try/an attempt at) + last letter of(latest in) “ontology” ].
Defn: …, that is, the philosophy of yoga, a school of Hindu philosophical traditions.
Answer: .., perhaps more of a practitioner than a student.
9 TV show repulsed nonsense and hoax – how peculiar (6,3)
DOCTOR WHO : Reversal of(repulsed) [ ROT(nonsense) plus(and) COD(a hoax/joke) ] + anagram of(… peculiar) HOW.

10 Something in Harry Potter – book having capacity to stay ahead of criticism (10)
BROOMSTICK : B(abbrev. for “book”) plus(having) ROOM(capacity/space to accommodate) plus(to stay ahead of) STICK(severe criticism/scolding).
11 Mass complaints in radio broadcast (4)
SIZE : Homophone of(… in radio broadcast) “sighs”(audible exhalations expressing tiredness, that could be interpreted as complaining).
Defn: …, used non-technically to denote the extent/overall magnitude of something.
12 Weak device for transporting policeman finally engaged in one detective story (9)
WHODUNNIT : W(abbrev. for “weak” in quantum physics, in relation to the weak interaction between subatomic particles)+ HOD(a device for transporting bricks and other building materials) + [ last letter of(… finally) “policeman” contained in(engaged in) UNIT(a singular quantity/one) ].
Whodunn presented the TV mystery game show? You Know Who, of course.

14 Was first to go round current shop (4)
DELI : Reversal of(… to go round) LED(was first) + I(symbol for electrical current in physics).
15 Ocean feature getting cut, cut, cut? That’s to have added resonance (2-4)
RE-ECHO : “reef”(a ridge of coral, rock or sand just above or below the surface/a feature of the ocean) minus its last letter(getting cut) + “chop”(to cut, with an axe or a chopper, say) minus its last letter(cut).
17 Devoted attendant paid in the end, after rejection (6)
LAPDOG : PD(abbrev. for “paid”/bill settled) contained in(in the) reversal of(…, after rejection) GOAL(an end/an objective as in “a means to an end”).
19 Line out of fashion is a pain for the observer (4)
STYE : “l”(abbrev. for “line”) deleted from(out of) “style”(fashion/a distinctive appearance).
Defn: …/eye.
21 Insensitive sort? My dear chap, ignoring an American is dreadful (9)
PACHYDERM : Anagram of(… is dreadful) “My dear chap” minus one(ignoring an) “A”(abbrev. for “American”).
Defn: …, as a result of having a thick and impervious skin.
24 Many students are confused by term in algebra (4)
YEAR : Anagram of(… confused) ARE placed after(by) Y(symbol/term for an unknown quantity in algebra).
Defn: …/a group of students in the same academic year.
25 About to include Republican in vernacular description of her opponents? (10)
DEMOCRATIC : [ CA(abbrev. for “circa”/about, applied to event dates) containing(to include) R(abbrev. for “Republican”, a member of the Republican political party) ] contained in(in) DEMOTIC(the vernacular/colloquial speech).
Defn: …, that is, a Republican’s opponents.
27 Navigational device – there – surrounded by a flashing light (9)
ASTROLABE : LA(a form of “lo!”/”oh look!”/”there!”, as in “La! What a beauty!”) containing(surrounded by) [A + STROBE(short for a stroboscope/a flashing light) ].
Defn: …, in use before being replaced by the sextant.

28 Undefined quantity in organisational symbol (4)
LOGO : LOG O(log 0, with O representing 0/logarithm of 0/an undefined quantity – the equation N (any number) to the power of X=0 cannot be solved for X).
Down
1 Plunder picked up by boater? One intends to return things (8)
BORROWER : Reversal of(… picked up, in a down clue) ROB(to plunder/to steal goods) plus(by) ROWER(one who rows a boat/a boater, perhaps, as in one who drives a truck is a trucker).
2 Perform unaccompanied, soprano having left for dancing partner (6)
GIGOLO : GIG(to perform at a gig/a live performance by a musician or group) + “solo”(unaccompanied/by oneself) minus(… having left) “s”(abbrev. for “soprano”).
Defn: A male escort paid or financially supported by a woman to be her partner at dancing, but often not just restricted to dancing.
3 Period in month that is not fully realised (4)
IDES : “Id Est”(abbreviated to ie./that is) minus its last letter(not fully realised).
Defn: In the ancient Roman calendar, a day falling roughly in the middle of the month
4 Party dropping first court case (6)
ACTION : “faction”(a party/a group within a larger group) minus its 1st letter(dropping first).
5 Never deviating, drink emptied daily, served with ice (4-6)
ROCK-STEADY : TEA(a drink) + “daily” minus all its inner letters(emptied …) placed below(served with, in a down clue) ROCKS(like “ice”, slang for “diamonds”).
6 Broadcast picked up in time: start of debate covering both opinions (3-5)
TWO-SIDED : Reversal of(… picked up, in a down clue) SOW(to broadcast/to scatter seed over a wide area) contained in(in) TIDE(“time period” in Old English/currently used in combination, to mean “a time” or “season”, as in “Christmastide”) +1st letter of(start of) “debate“.
7/18 Scarecrow left on stick, attracting insect, with zero going around up top (6,8)
WORZEL GUMMIDGE : [ L(abbrev. for “left”) placed above(on, in a down clue) GUM(to stick using, well, gum/glue) plus(attracting) MIDGE(an insect, viz. a small two-winged fly) ] placed below(… up top, in a down clue) [ W(abbrev. for “with”) + anagram of(… going around) ZERO ].
Defn: … who came to life in a series of children’s books.

13 Not expected and possibly profound, eh? (7-3)
UNHOPED-FOR : Anagram of(possibly) PROFOUND, EH.
16 Cross in middle of week over old car, producing curse (8)
EXECRATE : X(a cross-shaped symbol) contained in(in) middle 2 letters of(middle of) “week” placed above(over, in a down clue) CRATE(an old and dilapidated car).
18 See 7
20 Climbing expert brought in first boot accessory (6)
TOECAP : Reversal of(Climbing, in a down clue) ACE(an expert/one proficient in something) contained in(brought in) TOP(first/ahead of everyone else).
22 Marsupial heading off after end of epic fight (6)
COMBAT : “wombat”(an Australian marsupial) minus its 1st letter(heading off) placed below(after, in a down clue) last letter of(end of) “epic“.
23 Facilitate sound but not light, ultimately (6)
ENABLE : “tenable”(sound/able to be defended against objection) minus last letter of(…, ultimately) “light“.
26 Treat for dog, with dog getting energy for nothing (4)
CHEW : “chow”(a sturdy Chinese breed of dog) with “e”(symbol for “energy” in physics) replacing(for) “o”(letter representing 0/nothing).
Defn: Something for a dog who, like all dogs, naturally loves to chew.
I did note that I might not be appearing on Fridays in July…
On a personal note: Whodunnit ran a competition in TV Times (over 40 years ago), with a detective story to be solved. Successful solvers were to go into a draw for the chance to appear on the programme. In the end, they only just had enough correct entries to cover the number of episodes, so everyone got a go. And, yes, one of them was me.
Got WORZEL GUMMIDGE & DOCTOR WHO quite early on so was expecting a Jon Pertwee theme. Eventually saw his name appear along the bottom. Didn’t know WHODUNNIT programme – nice bit of extra info on this from our setter.
The mathematician in me did like LOGO. Quite a few clues took some teasing out, which is how I like it, with GIGOLO my LOI. LA for “there” seemed a bit unusual but makes a change from the US city or the French article.
Nice one Phi. Thanks also to scchua for the comprehensive blog.
… Well… obviously I did not receive the memo…
We didn’t spot the nina till we’d finished but then we realised what was afoot and were mildly surprised that there were only three themed entries. Apart from the theme we too liked LOGO. IDES was one of our favourites too.
Thanks, Phi and scchua.
The proof lies herein.
https://www.imdb.com/title/tt0069656/fullcredits/?ref_=tt_ov_st_sm
Next thing, he’ll be claiming he was on Mastermind 🙂
Gwep @7: oh, crivvens, I never knew that…
@5, I meant, in an unexpected moment of self-referential mistyping.