1. GB due, amazingly, to do software fix (5)
DEBUG - anagram (amazingly) of GB DUE.
4. Very good to interrupt boss — or foolish (6)
STUPID - very good (PI - pious) inside boss (STUD).
9. Carry on endlessly with dog to get bird (7)
WAGTAIL - carry on endlessly (WAG)e, dog (TAIL/follow). Carry on took some time - it is to engage in but in the sense of carrying on for some time - Collins has to engage in or carry on.
10. Husband leaving local church area for capital city (5)
PARIS - husband (H) leaving local church area (PARIS)h.
11. Share of proceeds is not entirely attractive (3)
CUT - not entirely attractive (CUT)e.
12. Clearing out records, as it might be, is corrupting (8)
DEFILING - filing is to put records into files so DE-FILING might be to clear them out.
15. Tell everything about the accident in the coffee-house? (5,3,5)
SPILL THE BEANS - accidental spilling of coffee beans.
17. Last tramline is buckled (8)
TERMINAL (of, being, or situated at an end, terminus, or boundary) - anagram (buckled) of TRAMLINE.
18. Dessert that is served after last of soup (3)
PIE (e.g. apple) - that is (IE) after sou(P).
20. Expect article will be penned by a comedian (5)
AWAIT - article (A) penned inside a (A) and comedian (WIT).
22. English literature is initially terribly highbrow? (7)
ELITIST - English (E), literature (LIT), is (IS), (T)erribly.
23. Dozy agent surrounding sheltered area (6)
SLEEPY - agent (SPY) surrounding sheltered area (LEE).
24. Hotel residents having no time for estimate (5)
GUESS - hotel residents having no time (GUES)t(S).
1. Blue feathers getting tossed (8)
DOWNCAST (not an anagram, as I thought at first, of feathers) - feathers (DOWN), tossed (CAST).
2. River coming away from sunny bay (5)
BIGHT - river (R) coming away from sunny (B)r(IGHT). E.g German Bight which comes between Fisher and Humber in the shipping forecast.
3. Piano’s not entirely inadequate, winning all the trophies (5,4)
GRAND SLAM - piano's (GRANDS), not entirely inadequate (LAM)e.
5. Hint that’s a lot of rubbish? (3)
TIP - rubbish tip.
6. Dangerous fish? Greek character fled, with cry of surprise (7)
PIRANHA - Greek character (PI), fled (RAN), cry if surprise (HA!).
7. Course in Oxford is hard (4)
DISH (not Isis but a course of a meal) - answer in the clue - Oxfor(D IS H)ard.
8. Dicey moment if heads of finance house are discovered in error (11)
CLIFFHANGER - if (I needed the F from defiling to see this) and the heads of (F)inane and (H)ouse inside error (CLANGER).
13. Science centre heading off effective classifying (9)
LABELLING - science centre (LAB), heading off effective t(ELLING).
14. Dangerous material? Most suitable to engage in a plea for help (8)
ASBESTOS - most suitable (BEST) inside a (A) and plea for help (SOS).
16. Repeat it before scoundrel appears in middle of week (7)
ITERATE - it (IT) before scoundrel (RAT) inside middle of w(EE)k.
18. Eg two or three runs I’m to get in physical exercise (5)
PRIME (numbers) - runs (R) and I'm (IM) inside physical exercise (PE).
19. Girl showing style, not the first (4)
LASS - style, not the first c(LASS).
21. Best spinner (3)
TOP - double definition - highest quality/spinning top.