Across:
8 Tailor (tailor/make alterations = CHANGE) opened 15 across, for the most part (opened 'changed one's tune' = 'changed' without 'd' = CHANGE),
9 Society (-S-) in Lincoln (Abe Lincoln = A-BE-) with drunks (sots) knocking back (= reverse indicator) (sots = -STOS) (ASBESTOS) a mineral laced with the harmful stuff (ASBESTOS),
10 Elected leader of (‘Elected’ leader of/first letter = -E) Brazil, for one (nut = TUN-), back (= reverse indicator) (TUNE) to pull some strings? (TUNE),
11 Greedy reaction (ITCHY PALMS) to 23 down (‘itch’ = ITCH-) with yard (-Y) and 24 across (‘palm’ = PALM-) with school (-S) (ITCHY PALMS),
12 Endless (e) disgrace (shame) (‘shame’ without ‘e’ = SHAM-) over (= reverse indicator) grandmother (nan) getting short (n) (‘nan’ without ‘n’ = na = -AN) (SHAMAN) with one who has a good knowledge of foreign spirits (SHAMAN)
14 Warnings (RED FLAGS), from the German (‘the’ German = der) upset (= reverse indicator) (der = RED) Betsy Ross (name of the American Flag) with the golden arrowhead (“The Golden Arrowhead” = the flag of Guyana)? (‘flag’ x 2 = FLAGS) (RED FLAGS),
15 Switched sides (CHANGED ONE’S TUNE) on LP – presumably that’s what DJ might have done? (CHANGED ONE’S TUNE),
18 Is detail (is detail) changed (= anagram indicator) (is detail = IDEALIST) by one looking for perfection? (IDEALIST),
20 Dobson flies (dobsonflies = INSECT), for example, with nicest (nicest) boss (= anagram indicator)? (nicest = INSECT),
22 Being more efficient (SAVING TIME) by presumably putting clocks in the safe (SAVING TIME),
24 Helpmate’s missing sheet (‘helpmates’ missing ‘sheet’ = lpma = PALM) – the one that should be on hand (PALM),
25 Trouble (= anagram indicator), albeit (albeit = BA-LETI-) around Lake (-L-) Como – the first (the ‘Como’ the first = -C) (BALLETIC) that’s typical of those taking steps to practise at The Bar (BALLETIC),
26 It’s (its = IT-S) about revolutionary (-CHE-) (ITCHES) irritations (ITCHES).
Down:
1 In spite of the fact (THOUGH), it's not easy (tough = T-OUGH) hanging on to hotel (-H-) (THOUGH),
2 Some time ago (ONCE) confined in detention centre (confined in ‘detentiON CEntre’ = ONCE),
3 Refusal to obey (DEFIANCE) English (-E-) and Irish men (Irish ‘men’ = fir) mostly (r) (‘fir’ without ‘r’ = -FI-) in Charleston (dance = D-ANCE) (DEFIANCE),
4 Soft white-grey material (TALC) buried by Oriental cockroach (buried by ‘orienTAL Cockroach’ = TALC),
5 Without a leader (m), mob (‘mob’ without ‘b’ = OB-) took a view (-EYED) (OBEYED) and followed (OBEYED)
6 Realises it (realises it) hurts (= anagram indicator) (realises it = ISRAELITES) the older people in The Middle East (ISRAELITES),
7 Mount (mount = TOM- -UN) Cook (= anagram indicator) overlooks exercise centre (gym) set up (= reverse indicator) (gym = -MY G-) (TOMMY GUN) by one fired for fighting during second World War (TOMMY GUN),
13 Turns up (= reverse indicator), everyone (all = -LLA) from institute (-I-) and academy (-A-) in New Zealand (NZ = -Z-N-) on short (e) term (name) (‘name’ without ‘e’ = nam = MAN-) (MANZANILLA) gets a drink! (MANZANILLA),
14 Cross (rood) unfinished (d) (‘rood’ without ‘d’ = ROO-) street (-ST) (ROOST) to stay in for the night (ROOST),
16 Retreat (HIDEAWAY) from pelt (pelt/skin = HIDE-) found on a (-A-) thoroughfare (-WAY) (HIDEAWAY),
17 A sudden undesirable phenomenon (EPIDEMIC) produced by barbaric (= anagram indicator) Medici (Medici = -IDEMIC) after (= positional indicator) revolting (= reversal indicator) exercise (PE = EP-) (EPIDEMIC),
19 Bars (bars = INGOTS) popular (IN-) with leads going on the stage (leads/first letters of ‘Going On The Stage’ = -GOTS) (INGOTS),
21 Liberator left Liberal Democrat (‘Liberator’ left ‘Liberal Democrat’ = ldemca = CALMED) and settled down? (CALMED),
23 Two-thirds of 26 across (two thirds of ‘ITCHes’ = ITCH) getting allergic reaction (ITCH),
24 Stuff (PACK) in jam? (PACK).