My goodness, I found this 19dn. It is true that the building work next door has been somewhat vexing and that I haven't had time since last Tuesday to enjoy any cryptic work-outs, but 18 minutes makes this a toughie in my book. I simply 'wasn't at the races' with so many clues so hats off to Wurm and I hope you all fare better than I.
1. A scale showing quality (6)
AMOUNT - a (A), scale (MOUNT - as in scale a fence/mountain).
5. Harry rowed around one strange fish (6)
WEIRDO - anagram (Harry) of ROWED around one (I). I didn't see the anagram for a long time.
8. Drunken pirate provided opening shot (8)
APERITIF - anagram (drunken) of PIRATE, provided (IF). What a definition!
9. On reflection, humour brings unpleasant fate (4)
DOOM - humour - mood (he was in a bad humour) backwards - DOOM.
10. Composer seen in November gathering (4)
BERG - seen in Novem(BER G)athering. Alban Maria Johannes Berg 1885 - 1935.
11. Obscure period without Sun and Times? (4,4)
DARK AGES - a period without light and culture is a dark age. Ho Ho. Well - I enjoyed it. Our friend The Rotter has, rightly, pointed out that it parses as without Sun (DARK), times (AGES).
12. Mock name for the common herd (6)
RAGTAG - mock (RAG), name (TAG). I've heard of a ragtag collection being a motley crew but struggled to see it as common herd.
14. Street vendor one clearing throat? (6)
HAWKER - double definition.
16. Back winner (8)
Champion - double definition.
18. Capital generated in two American states (4)
RIGA - states of Rhode Island (RI) and Georgia (GA).
20. Some para inevitably drops from sky (4)
RAIN - some of pa(RA IN)evitably.
21. He has dog rolling barrel (8)
HOGSHEAD - anagram (rolling) of HE HAS DOG.
23. Small son retaining very French accent (6)
STRESS - small (S) and son (S) holding 'very' in French (TRES).
24. Delegate stuck in river (6)
DEPUTE - stuck (PUT - if you stick something somewhere you put it there in a casual way) inside river (DEE). Delegate/depute as a verb.
2. Mike cored apple in wood (5)
MAPLE - mike (M), apple without its core (APpLE).
3. Honourable leading Tories? (7)
UPRIGHT - leading (UP). Tories (RIGHT).
4. Dram from non-drinker getting round in (3)
TOT - non-drinker (TT) getting round (O) inside.
5. Narrow victory keeping father excited (5-4)
WAFER-THIN - victory (WIN) holding an anagram (excited) of FATHER.
6. Nation appears in Kremlin dialogue (5)
INDIA - inside Kreml(IN DIA)logue.
7. See church featured in OED is replaced (7)
DIOCESE - church (CE) inside an anagram (replaced) of OED IS.
11. Say Snoopy battles Red Baron in these? (9)
DOGFIGHTS - Snoopy is an example of (say) a DOG, battles (FIGHTS). The Peanuts cartoon featured Snoopy battling the Red Baron which makes this the stand-out COD for me.
13. Essentially a nasty threat (2,5)
AT HEART - a (A), anagram (nasty) of THREAT.
15. Love having dispute about vessel (7)
WORSHIP - dispute - row - backwards - WOR, vessel (SHIP).
17. Chopped meat Charlie dumped in pit (5)
MINCE - Charlie (C) inside pit (MINE).
19. Tissue transplant is hard work (5)
GRAFT - double definition.
22. Lord to leave with daughter (3)
GOD - leave (GO) with daughter (D).