Thank you to Filbert (for this penultimate IoS, I believe). Definitions are underlined in the clues.
Across
1. Huge margins on suit paid for by Charles (8)
STONKING : 1st and last letters of(margins on) “suit” + ON(is a treat from/paid for by someone, as in “dinner’s on me”) + KING(currently, title of Charles III of the UK).
Defn: Slang for ….
6. Foul-mouthed daughters wear your clothes (6)
SWEARY : Hidden in(… clothes) “daughters wear your”.
9. Excited, since heading for Greece (4)
AGOG : AGO(from a specified moment in the past/since) + 1st letter of(heading for) “Greece”.
10. Software glitch on drone registered in 2001 inverted cant (10)
HUMBUGGERY : [ BUG(a flaw in a computer program or system/software glitch) placed after(on) HUM(a low-frequency steady noise/drone) ] + reversal of(… inverted) Y-REG(refering to cars registered in the UK between March 2001 and August 2001, having Y as a prefix on their number plates).
Defn: …/insincere talk.
11. Assassin‘s not at all sweet unless on vacation (6)
BRUTUS : BRUT(French for “dry”, used to refer to champagne and sparkling wine that does not taste sweet) + “unless” minus its inner letters(on vacation).
Answer: One of the assassins of Julius Cesar.
13. Foul tide swirling in the deep (8)
OUTFIELD : Anagram of(… swirling) FOUL TIDE.
Defn: …/the area of a cricket field that is relatively far from the pitch.
14. Spouse off booze taking single small beer outside, then another (4,6,4)
ONE’S BETTER HALF : TT(abbrev. for “teetotaller”, one who abstains totally from alcohol/off booze) contained in(taking … outside) [ ONE(single) + S(abbrev. for “small”) + ] plus(then) HALF(a half-pint of beer or, per the clue, another small beer).
16. London Eye? (8,6)
SHERLOCK HOLMES : Cd. Reference to an investigator/eye based in London.
20. Medical professional with Tesla breaking heart for little red sportscar? (8)
CORVETTE : [ VET(short for “veterinary surgeon”, a medical professional dealing with animals) ps(with) T(abbrev. for “Tesla”, the unit of magnetic flux density, in physics) ] contained in(breaking) CORE(the heart/centre of something).
1st generation Chevrolet Corvette:
… and “Little red corvette” is a song by Prince.
22. Puzzle for UK a Kuwaiti has smuggled over (6)
KAKURO : Hidden in(… has smuggled) reversal of(… over) “for UK a Kuwaiti”.
Puzzle and solution:
23. Huge growth ruined Saudi golf, ending in disaster (7,3)
DOUGLAS FIR : Anagram of(ruined) [SAUDI GOLF + last letter of(ending in) “disaster” ].
25. Worry line on neck (4)
FRET : Double defn: 1st: …/be anxious; and 2nd: …/one of the small metal strips across the fingerboard of a stringed musical instrument like the guitar.
26. Offer to release new tune in advance (6)
PRESET : “present”(to offer/give) minus(to release) “n”(abbrev. for “new”).
Defn: …/to set a control at a certain point or level in advance, eg. the station to be received on a radio.
27. Music festival in Morpeth’s flourishing (3,5)
THE PROMS : Anagram of(… flourishing) MORPETH’S.
Down
2. Bug terribly hot it admits, being drawn to flame (5,4)
TIGER MOTH : GERM(an organism such as bacteria and virus that can cause illness/bug) contained in(… admits) anagram of(terribly) HOT IT.
Answer: A member of the moth family, insects which are attracted to light.
3. Warmer in bed in close relationship (7)
NIGHTIE : NIGHT(close to/near, in time or space) + TIE(a connection/relationship).
4. Sort of angle without apex (3)
-ISH : “fish”(to angle/catch fish with hook and line) minus(without) its 1st letter(apex, in a down clue).
Defn: …/a suffix for “somewhat”/”approximately”.
5. Wellington perhaps back from Waterloo, tense beneath lady’s fingers (7)
GUMBOOT : [ Last letter of(back from) “Waterloo” + T(abbrev. for “tense” in grammar) ] placed below(beneath, in a down clue) GUMBO(American for okra/lady’s fingers).
Gum rubber boots:
6. Spooner’s slice will astound participant in rally (11)
SHUTTLECOCK : Spoonerism of(Spooner’s) [“cut’ll”(contraction of cut will/slice will) “shock”(to stound/surprise).
Answer: The object hit back and forth in a rally in a game of badminton.
7. People Scot might not like trashing his glen (7)
ENGLISH : Anagram of(trashing) HIS GLEN.
Defn: …, refering to the Anglo-Scottish Wars, a series of conflicts between England and Scotland.
8. Two rivers in the country (5)
RURAL : [R(abbrev. for “river”) + URAL(river flowing through Russia and Kazakhstan)](two rivers).
12. Farm out west of Chippewa on land with reserve to the north (11)
SUBCONTRACT : [ 1st letter of(west of) “Chippewa” + ON + TRACT(a large piece of land) ] placed below(with … to the north, in a down clue) SUB(short for “substitute”, a reserve/one who stands in for another).
Defn: …, when a main contractor for a project contracts another contractor to do part of the project work.
15. Ray reportedly sleeps with a smile (5,4)
LASER BEAM : Homophone of(reportedly) “lays”(a non-standard term for “lies”/puts oneself in a reclining position such as when sleeping) + A + BEAM(a broad smile).
17. Video game company very upset faced with artist’s terrible effects (7)
RAVAGES : Reversal of(… upset, in down clue) [ SEGA(a Japanese multinational video games and entertainment company) + V(abbrev. for “very”) ] placed below(faced with, in a down clue) RA(abbrev. for “Royal Academician”, a member of the Royal Academy of Arts/an artist).
18. Castle suitable for jerks? (4,3)
KEEP FIT : KEEP(the main tower of a medieval castle) + FIT(suitable for that purpose).
Answer: …, also, “keep-fit” is the activity of keeping your body in good condition by doing physical exercise/an old-fashioned term for the latter is “jerks”.
19. Result in: Spooner’s not genuine choice for James Bond (4,3)
MAKE FOR : Spoonerism of(Spooner’s) [ “fake”(not genuine/sham) + “Moore”(Roger, an actor who was once the choice to play James Bond in the movies).
Answer: …/bring about an outcome/result in, as in “A more comprehensive analysis would make for a better explanation”.
21. Musk perhaps grim with nothing on (5)
ODOUR : DOUR(stern-looking/grim) placed below(with … on, in a down clue) O(letter representing 0/nothing).
Defn: An example of which/perhaps is musk/the smell of musk, a strong-smelling earthy substance used in perfumes, or a similar smell.
24. Retrospectively wish you could change uniform on guards? (3)
RUE : U(abbrev. for “uniform”) contained in(… guards) RE(with reference to/on).
Defn: …/to regret what has happened or been done.
Really clever, really enjoyable, thanks Filbert. Took a punt that 2001 equals Y reg in the UK, but everything else parses without checks, which is rare in itself. Great stuff.
Lovely puzzle, thanks both (scchua, in 2d you’ve underlined a bit of the wp; the def, as per your explanation, is ‘being drawn to flame’).