12 Bad person, no saint, that may offer something for the writer (6)
INKPOT - st(INKPOT) - yes, but not since 1950

14 Agent wants Alli to attract crowd at match (8)
DELEGATE - some footballer I've never heard of + GATE

15 Gone after final wish for something that's far too expensive (3-3)
RIP-OFF - Requiescat In Pace + OFF

17 Give sign of respect, having received shocking treatment (6)
BESTOW - EST inside BOW; I tried to make 'kowtow' work for a while; EST is Electric Shock 'Treatment' - the shocking thing is that it is still administered!

20 Go wrong in atria, wrong environments for plants (8)
TERRARIA - ERR inside (atria)*

22 Aztec leader being deposed sadly with soldiers entering - led by me? (6)
CORTEZ - OR (other ranks) inside (ztec)*

23 No promises to be broken? It sounds the wrong way round (10)
SPOONERISM (no promises)* - I haven't seen this before, but it can't be the first instance

24, 10 Respond to demand of traffic cop in jumper (8)
PULLOVER - obvious

25 Something a bit hairy about community we‘re leaving? Get better informed! (4,2)
WISE UP - WISP around EU - he's being fashionable again, referring to Brexit

26 Looked disgusted, given ghastly publicity about church (8)
GRIMACED - CE inside GRIM + AD

1 Trophy item that could be unobtrusive, but possibly not needed (8)
SOUVENIR - (ubrusive)* unobtrusive without 'not'

3 Lord half-cut in tartan? Maybe he can't grow up (6)
KIDULT - DU(ke) inside KILT

2, 24 Being fanciful about mountain climbing's code of ethics? (4,4)
FAIR PLAY - ALP inside FAIRY

4 Pasquale boring? Aged brainy bits! (8)
MEDULLAE - ME + DULL + AE - alternate letters of 'aged'?

5 Severe critic to have fresh go at racist (10)
CASTIGATOR - (go at racist)*

6 One American going up in plane, a man of habit (6)
JESUIT - I US rev. inside JET

8 Fractions? That‘ll finish classes off (6)
THIRDS - In a Honours degree, and railway, formerly, first, second, and ...

13 Style one noticed in remuneration? Cash now only in £50 notes (5,5)
PAPER MONEY - PERM + ONE inside PAY

16 Supporting medieval buildings permanently (3,5)
FOR KEEPS - obvious

18 Little stories about Beckham‘s innermost character in magazines (8)
WEEKLIES - K inside WEE LIES

19 Songbird not weak at heart growing bigger (6)
WAXING - WAX(w)ING - apparently w is an abbreviation for weak?

21 Life of religious minister English Society's elevated (6)
ESPRIT - not yet understood

22 Pledge from left-winger falling short over time (6)
COMMIT - COMMI(e) + T

24 See 2
PLAY FAIR