1. Work at school on ballet about Queen’s supreme importance (13)
PREPONDERANCE – work at school (PREP), on (ON), ballet (DANCE) about Queen (ER).
8. Consume as a joke, reportedly (6)
INGEST – homophone (reportedly) of as a joke – in jest.
9. Go wrong with delivery run (6)
ERRAND – had to force myself to complete the parsing here as ‘errant’ wanted to be entered. Go wrong (ERR), with (AND).
10. Charge when admitting first of tourists for outdoor event (4)
FETE – charge (FEE) admitting (T)ourists.
11. One no trump concluded as planned (8)
INTENDED – one (I), no trump (NT) (or maybe 1NT is bridge notation), concluded (ENDED).
12. Sacred emblem louts destroyed (5)
LOTUS – anagram (destroyed) of LOUTS. Symbol of the divine in humanity, apparently.
13. Head of old Oxford debating society abandoning university (5)
ONION – old (O), Oxford debating society u(NION) – without u for university. The Oxford Union have been hosting world-famous debates and speakers since 1823.
15. Aver bias, terribly harsh (8)
ABRASIVE – anagram (terribly) of AVER BIAS.
17. Stringed instrument in sack, we hear (4)
LUTE – homophone (we hear) of sack – loot.
19. Perfect place in the middle of Luton, one in quiet area (6)
UTOPIA – l(UTO)n, one (I) in quiet area (P A).
20. Additional despatch cut by company (6)
SECOND – despatch (SEND) cut by company (CO).
21. Changing into neat Roman embroidery (13)
ORNAMENTATION – anagram (changing) of INTO NEAT ROMAN.
DOWN
2. Fury about new diversity (5)
Range – fury (RAGE) about new (N).
3. Gift posted again under pressure (7)
PRESENT – posted again (RE-SENT) underneath pressure (P).
4. Brazil perhaps has old king beheaded (3)
NUT – King c(NUT) the Great.
5. Put in bar with English? Terminal (9)
ELECTRODE – put in (ELECT – an MP), bar (ROD), English (E).
6. Month working in protective garment (5)
APRON – month (APR), working (ON).
7. Is youngster allowed in works restaurant? (7)
CANTEEN – is youngster allowed (CAN TEEN).
11. Begin with soldier brought up in home country (9)
INSTIGATE – soldier (GI – brought up=IG) inside home (IN) and country (STATE).
12. Bolster failing target for fishermen? (7)
LOBSTER – anagram (failing) of BOLSTER.
14. Villa lick City with no wingers? Criminal (7)
ILLICIT – v(ILL)a l(IC)k c(IT)y no wingers/ends.
16. Greek character in theatrical Phaedra (5)
APLPHA – in theatric(AL PH)aedra.
18. Beginning to try a new turn in dance (5)
TANGO – (T)RY, a (A), new (N), turn (GO).
20. Place for filming small alien film (3)
SET – small (S), alien film (ET).
https://english.stackexchange.com/questions/115308/whats-the-origin-of-the-saying-know-your-onions
Like know the ropes, the onions are things in the outside world which are known about through a process of experience/education.
LOI Onion
17A COD
This is from:
https://www.oed.com/page/pgwodehouse/P.G.+Wodehouse+in+the+OED
There’s also:
https://lingomash.com/slang-meanings/41744/slang-meaning-of-off-one-s-onion