Across:
8 Dog (FOLLOW) track? (FOLLOW),
9 Call (call = CAL-L-) over copper (-CU-) from America (-US) (CALCULUS) offering a mathematical method of reasoning (CALCULUS),
10 Ease off the sea miles (‘ease’ off the ‘sea miles’ = mils = SLIM) and get into shape (SLIM),
11 Couple of views (eye + witness = EYEWITNESS) in The Observer? (EYEWITNESS),
12 Sounds like advice to the jockey (STAY UP) to postpone retirement (STAY UP),
14 Bit of a yarn (lint = LI-NT) about football (-GAME-) (LIGAMENT) is close to the bone (LIGAMENT),
15 Organic material (NATURAL) in a number of researchers’ fields (SCIENCES) (NATURAL SCIENCES) studied by chemists and physicists (NATURAL SCIENCES),
18 In a stew all over north Africa (COUSCOUS), as two sets of military takeovers (coups x 2) both lack power (p) (‘coups + coups’ without ‘p’ = COUSCOUS),
20 Oh (“oh” = “O” = OXYGEN), sounds like (= homophone indicator) it’s in the tent (OXYGEN tent),
22 Disguise (cover) headless (c) (‘cover’ without ‘c’ = OVER-) ghost (-SHADOW) (OVERSHADOW) or dwarf (OVERSHADOW),
24 The essence (essence/heart = PITH) of what’s locked up in decrepit household (what’s locked up in ‘decrePIT Household = PITH),
25 Larks in a (larks in a = SRI LANKA) wild (= anagram indicator) island country (SRI LANKA),
26 Drifted (ROAMED) from area (-A-) in European city (RO-ME-) towards bad end (‘baD’ end = -D) (ROAMED).
Down:
1 Settle down (COOL IT) with trendy (COOL) comedian (wit) needing no introduction (w) (‘wit’ without ‘w’ = IT) (COOL IT),
2 A feather in one’s cap (plume) finishing off (e) (‘plume’ without ‘e’ = PLUM-) claret (PLUM),
3 Small (S-) tiny (tiny/wee = -WEE-) port (-P-) consumed in brew (tea = -T -EA) (SWEET PEA) produced in the garden (SWEET PEA),
4 Spot (AREA) found within the cardiac region (found within ‘the cardiAC REgion’ = AREA),
5 Is winging it (FLYING), as figurehead (‘Figure’ head = F-) is not telling the truth (-LYING) (FLYING),
6 The proceeds of risible bread-maker? (FUNNY MONEY) Anyone rolling out their own dough will get this! (FUNNY MONEY),
7 New (N-) sauce in (sauce in = -UISANCE) salad (= anagram indicator) (NUISANCE) is cause of annoyance (NUISANCE),
13 Me- (YOURS TRULY) -me? (YOURS TRULY),
14 On the radio (= homophone indicator), composer (‘Liszt’ = ‘list’ = LIST-) with One Direction (south = -S) (LISTS) produced the records (LISTS),
16 Plant (ALOE) opens very early, ripens and (opens ‘Very Early Ripening And’ = VERA) (ALOE VERA) produces cosmetic ingredient (ALOE VERA),
17 Most of (w) the squabble (most of ‘row’ = -RO-) occupying popular (in = I-N) producer’s composition (work = -WORK) (IRONWORK) is typical of what you get with The Gate (IRONWORK),
19 Present (ON HAND) from Ring, for example? (ring is ON HAND),
21 Open letter dismisses plot (‘open letter’ dismisses ‘plot’ = eneter = ENTRÉE), of course (ENTRÉE),
23 Included in formulae developed (included in ‘formuLAE Developed’ = laed = DEAL) setting up (= reversal indicator) work in the drugs trade (DEAL),
24 Both sides of the peppered steak (both sides of the ‘PEppered steAK’ = PEAK) for one of the widows going on ahead? (v-shaped growth of hair is called a widow’s PEAK).