Across:
8 Wink (SIGNAL) from Bill (bill/notice = SIGN-) associated with American (-A-) First Ladies (first 'Ladies' = -L) (SIGNAL),
9 A (A-) partner (-MATE-) from Russia (RU) reading up (= reverse indicator) (RU = -UR-) on introductory synopsis (introductory ‘Synopsis’ = -A) (AMATEURS) to Corinthians (AMATEURS),
10 Clips (CUTS) of leaders cracking under the strain (leaders ‘Cracking Under The Strain’ = CUTS),
11 Put away (IMPRISONED) papers (ID papers = I-D) about Mike (-M-) and Con’s place (con/criminal’s place = -PRISON-) that’s close to home (close/end to ‘homE’ = -E-) (IMPRISONED),
12 Mafia leader (Don = D-ON) oversees scrap (-RAG-) (DRAGON) with mouthy arsonist, if the myths are to be believed (DRAGON),
14 Key (music key C-) to German address (German “Mr” = -HERR-) that is (i.e. = -IE-) south (-S) (CHERRIES) of the orchard (CHERRIES),
15 How to give yourself a big head perhaps (BROADEN ONE’S MIND)? Take an interest in New Tricks! (BROADEN ONE’S MIND),
18 Spreads (BROADENS) and starter for 15 across (starter of ‘broaden one’s mind’ = BROADEN-) at school (-S) (BROADENS),
20 Jazz (= anagram indicator) singer (singer = RESIGN) gets to bow out (RESIGN),
22 Boring (FLAT) jockey (= anagram indicator) caring (caring = RACING) (FLAT RACING) for the Arabians, of course (FLAT RACING),
24 One of the deposits (MARL) at the banks making your toes curl? (at the banks ‘MAking your toes cuRL’ = MARL),
25 Defended opinion of (ASSERTED) fool (ASS-) in hospital (-ER-) – the one loving Elvis (fan of Elvis = Teddy Boy = -TED) (ASSERTED),
26 Older relative (NAN-) gets set (set) up (= reverse indicator) (set = -TES) (NANTES) in French city (NANTES).
Down:
1 A glass of whiskey (LIQUOR) for all sorts (Liquorice All Sorts) without ice? ('liquorice' without 'ice' = liquor = LIQUOR),
2 Individuals (ONES) at the centre of 15 across (at the centre of ‘broaden one’s mind’ = ONES),
3 Russell (George William Russell = AE = A-E) welcomes lines (-LL-) to one (-I-) party (African National Congress = -ANC-) (ALLIANCE) in coalition (ALLIANCE),
4 Group (group/side = CAMP) cut initially (‘Cut’ initially = C-) from Band Aid (band aid is an -AMP) (CAMP),
5 March (MAR-) in (-IN-) Spain (-E) (MARINE) relates to the sea (MARINE),
6 Endless (n) fruit (‘melon’ without ‘n’ = MELO-) from Madras (Madras) freshly (= anagram indicator) (Madras = -DRAMAS) (MELODRAMAS) produced in the abbey (MELODRAMAS in The Abbey Theatre),
7 Palm (TREE FERN) Ashe (ashe) overheard (= homophone indicator) (‘ashe’ = ‘ash’ = TREE) by member of Osmunda family (FERN is a member of Osmunda family) (TREE FERN),
13 Pledges (GUARANTEES) to use gear (use gear) jogging (= anagram indicator) (use gear = GUAR-EES) around a (-A-) new (-N-) square (-T- square) (GUARANTEES),
14 Considers abandoning Enid (‘considers’ abandoning ‘Enid’ = cosrs = CROSS) Bridge (bridge/span = CROSS),
16 It’s got nothing to do with Chicago (a city), for instance (nothing to do with city = RURAL-), having Number One (first = 1st = -IST) (RURALIST) produced by a lover of country (RURALIST),
17 Leads early Navajos (leads ‘Early Navajos’ = EN-) on the warpath (-RAGING) (ENRAGING) getting under someone’s skin (ENRAGING),
19 Speaks very highly (EXALTS) of headliner? (head ‘liner’ = -L-) In Texas (Texas) rave! (= anagram indicator) (Texas = EXA-TS) (EXALTS),
21 Writing (from the 3Rs = -R-) in great (great) novel? (= anagram indicator) (great = GAR-ET) (GARRET) An Attic Room With A View, perhaps! (GARRET),
23 Local paper (local “Independent” = INDO) produced by spin doctors internally (‘spIN DOctors’ internally = INDO),
24 Finishes 15 down (finishes ‘broaden one’s mind’ = MIND) with imagination (MIND).