Across:
8 Is not in salt mine ('is' not in 'salt mine' = altmne = MENTAL) and that's bonkers (MENTAL),
9 Clip of (e) stubborn type (mule) (‘mule’ without ‘e’ = -MUL-) consumed by upsetting (= reverse indicator) pollution (noise = E -SION) (EMULSION) in Mayo, for example (mayo/mayonnaise = a type of EMULSION),
10 Losing the head (c) with witchcraft (charm) (‘charm’ without ‘c’ = HARM) – in that lies the evil (HARM),
11 Those offering opinions (COLUMNISTS) to characters finishing 15 across (finishing ‘gossip columnists’ = COLUMNIST-) at school (-S) (COLUMNISTS),
12 They’re found under the roof (ATTICS) of a (A-) race (Isle of Man Tourist Trophy = -TT-) leader’s incredible car (leaders/first letters of ‘Incredible Car’ = -IC-) going at top speed (top ‘Speed’ = -S) (ATTICS),
14 Sum up (COUNT-) every second census (every second ‘cEnSuS’ = -ESS) (COUNTESS) for Earl’s wife (COUNTESS),
15 Woman presumably watching Stars at Night (GOSSIP COLUMNIST) from 21 down (‘gossip’ = GOSSIP) with one of those employed by someone who runs daily? (COLUMNIST employed by editor) (GOSSIP COLUMNIST),
18 Informers (rats) upset (= reversal indicator) (rats = STAR-) by those from school (-FISH) (STARFISH) lying about the beach (STARFISH),
20 Jazz (= anagram indicator) singer (singer = RESIGN) gets to bow out (RESIGN),
22 What the milliner created for classy individuals who haven’t been good (DUNCE’S CAPS) to the corner boys (DUNCE’S) from 24 down (‘caps’ = CAPS) (DUNCE’S CAPS),
24 Some laidback (= reverse indicator) incumbent sacked (some laidback ‘incumbenT SACked’ = tsac = CAST) by the players? (CAST),
25 Martin’s racy partner (ASTON- Martin) is (-IS-) first on hand (first on ‘Hand’ = -H) (ASTONISH) to amaze (ASTONISH),
26 Little rest (NAP-) for the family (-KIN) (NAPKIN) tied up in The Ring (NAPKIN in ring in the restaurant).
Down:
1 Make tracks (DEPART) for Playback (= reversal indicator) taped (taped = DEPA-T) around end of December (end of 'decembeR' = -R-) (DEPART),
2 The bit of physics (ATOM) in odd art forms (odd letters in ‘ArT fOrMs’ = ATOM),
3 Kitty (puss) retires (= reversal indicator) (puss = S-S UP) around lake (-L-) with frozen water (-ICE-) (SLICES UP) in sections (SLICES UP),
4 Sounds like (= homophone indicator) Trim (‘peel’ = ‘PEAL’) or Ring? (ring/chime = PEAL),
5 Bird (emu) heading off (e) (‘emu’ without ‘e’ = MU-) with hesitation (-UM-) to university (University of Ulster = -UU) (MUUMUU) in Hawaiian dress (MUUMUU),
6 Those helping (ASSISTANTS) to save someone’s skin (ASSIST-) from pesky insects (-ANTS) (ASSISTANTS),
7 Takes issue with (CONTESTS) a number of competitions (CONTESTS),
13 Trim (t) coach (instruct) (‘instruct’ without ‘t’ = INSTRUC-) on The Hill (-TOR) (INSTRUCTOR) produced guide (INSTRUCTOR),
14 Copyright (C-) for good books (Old Testament = -OT-) found in Louth (LH = -L-H) (CLOTH) by Georgette, for one (so named after French dressmaker Georgette de la Plant: georgette = crêpe material/fabric = CLOTH),
16 In bloom (OUT-) by stream (-BURST) (OUTBURST) producing an explosion of emotion (OUTBURST),
17 Pigs ruin (pigs ruin) building (= anagram indicator) (pigs ruin = UPRISING) – it’s what you’d expect from rebellious behaviour (UPRISING),
19 Gradually establish (INSTIL) from current (I-) – it’s (its) rising (= reverse indicator) (its = -STI-) in The Netherlands (-N-L) (INSTIL),
21 Apologists ignore a lot (‘apologists’ ignore ‘a lot’ = pogiss = GOSSIP) of tittle-tattle (GOSSIP),
23 Hotshot (ace = AC-E) ringing hospital (-H-) (ACHE) in distress (ACHE),
24 Trump’s (CAPS) kitten removed from septic tank? (‘kitten’ removed from ‘septic tank’= spca = CAPS).