Across:
1 A fair amount of (half) 14 across (half of 'ABSENTminded' = ABSENT) is not there (ABSENT),
4 Poser (CLUE-) from takeaway (take away = -LESS) (CLUELESS) has no idea (CLUELESS),
9 Jog someone’s memory (REMIND) playing (= anagram indicator) Minder (minder = REMIND),
10 EU dismissed by unsuitable (‘EU’ dismissed by ‘unsuitable’ = nsuitabl = ISTANBUL) old capital (ISTANBUL),
12 Built poor (built poor) structure (= anagram indicator) (built poor = POLITBURO) for The Party’s Committee (POLITBURO),
13 Some healthy meal (some ‘healTHY MEal’ = THYME) produced in the herb garden (THYME),
14 Sailor (Able Bodied seaman = AB-) from Cologne (scent), it’s said (= homophone indicator) (‘scent’ = ‘sent’ = -SENT), kept an eye on (MINDED) (ABSENT-MINDED) daydreaming (ABSENT-MINDED),
18 Really clever (SHARP AS A TACK), cutting (SHARP) answer (A-) from South African (SA = -S A) stud (stud/nail = TACK) (SHARP AS A TACK),
21 Raises (= reverse indicator) one (-I-) in casual (causal = lax = X-AL) area (A-) (AXIAL) in relation to Central Line (AXIAL),
22 Ten Romans (ten Romans) out of place (= anagram indicator) (ten Romans = ORNAMENTS) with decorations (ORNAMENTS),
24 It’s roughest (COARSEST) for Oscar (Oscar) travelling (= anagram indicator) (Oscar = COARS-) in the heart of Palestine (in the heart of ‘palESTine’ = -EST) (COARSEST),
25 Entirely (IN TOTO) produced by teachers’ (Irish National Teachers’ Organisation = INTO = IN TO-) principal (leading/top) briefly (p) (‘top’ without ‘p’ = -TO) (IN TOTO),
26 Charlie (C-) took stick in snooker hall (snooker extension stick = rest = -R-EST) over fool (-ASS-) (CRASSEST) who happens to be the most thoughtless (CRASSEST),
27 Flipping (= reversal indicator) pilot study (test = TSET-) gets most of (t) the set (‘set’ without ‘t’= -SE) (TSETSE) to fly (TSETSE).
Down:
1 Put out (AIR) sides of scallop (sides of 'ScalloP' = SP-) and flat fish (-RAY) (AIR SPRAY) that's preserved in a type of can (AIR SPRAY),
2 Child’s play (SIMPLE-) for school (-S-) is on time (-T) (SIMPLEST) and couldn’t be easier (SIMPLEST),
3 Group of nine (NONET) vermin out of environment (‘vermin’ out of ‘environment’ = onent = NONET),
5 Where you’d expect to find the mates potentially (LISDOONVARNA matchmaking festival) in Clare (LISDOONVARNA),
6 Rash (skin rash = EXANTHEMA) divorcee (EX-) has no answer (a) to what’s abhorrent to her? (anathema) (‘anathema’ without ‘a’ = -ANTHEMA) (EXANTHEMA),
7 In the early stages of life (EMBRYO) coming from Rome by (Rome by) train (= anagram indicator) (Rome by = EMBRYO)
8 Preserved (SALTED) bananas (= anagram indicator) lasted (lasted = SALTED),
11 Belvedere’s (Belvederes = SUMMER HOUSES) type of school (SUMMER school) diets (diets/parliaments = HOUSES) (SUMMER HOUSES),
15 Backing (= reverse indicator) popular (in = NI-) Number Ones (hits = -HI-TS) including short (t) record (list) (‘list’ without ‘t’ = -LIS-) (NIHILISTS) by The Extreme Sceptics (NIHILISTS),
16 Croatian (Croatian) novel (= anagram indicator) (Croatian = RAINCOAT) is perfect for that shower (RAINCOAT),
17 Most of (n) the film (‘skin’ without ‘n’ = SKI) pitch (SLOPE) (SKI SLOPE) has a tendency to go downhill (SKI SLOPE),
19 Describes limestone (CALCIC) in California (CA-) and summits of larger chains in Colorado (summits of ‘Larger Chains In Colorado’ = -LCIC) (CALCIC),
20 Valeric acid removes liver (‘valeric acid’ removes ‘liver’ = acacid = CICADA) bug (CICADA),
23 Looks after (MINDS) leopard from Promised Land (‘leopard’ from ‘Promised Land’ = misnd = MINDS).