Across:
1 Lie in wait for (AMBUSH) early morning (AM-) transport (-BUS-) with heroin (-H) (AMBUSH),
4 Builds up (AUGMENTS) to make greater (AUGMENT-) use (use) of the centre (s) (‘use’ of the centre = -S) (AUGMENTS),
9 Relative (AUNTIE) of American (A-) set free (-UNTIE) (AUNTIE),
10 In a state of flux (CHANGING) getting ready for a match (CHANGING),
12 Caught (C-) song (-A-RIA) reverberating around garden party (-FETE-) (CAFETERIA) in restaurant (CAFETERIA),
13 Function (EVENT) of English (E-) Channel? (channel/flue = -VENT) (EVENT),
14 Butting in (INTERRRUPTION) is no way to carry on (INTERRUPTION),
18 Detective (-DI-) from leading (‘from’ leading = -F-) women’s association (Irish Countrywomen’s Association = -ICA-) consumed by proposal? (motion = MO-TION) (MODIFICATION) That’ll need some getting used to! (MODIFICATION),
21 Neat staying off mediation (‘neat’ staying off/leaving ‘mediation’ = ‘mediation’ = mdiio = IDIOM), in a manner of speaking (IDIOM),
22 Gloats in a (gloats in a = NOSTALGIA) collection (= anagram indicator) out of sentimentality (NOSTALGIA),
24 First off (k), a measure of the speed coming in by sea (‘knot’ without ‘k’ = NOT) from what sounds like (= homophone indicator) a chain of islands (‘atoll = ‘AT ALL’) (NOT AT ALL) – don’t mention it? (NOT AT ALL),
25 Increasingly ignores aliens (‘increasingly’ ignores ‘aliens’ = incrgy = CRYING) turning on the waterworks (CRYING),
26 Exercises (PRESS-UPS) journalists’ (PRESS -) union (-U-) and pressure (-P-) on society (-S) (PRESS-UPS),
27 Decorative mask (VENEER) for disguise? (VENEER).
Down:
1 Lawless (ANARCHIC) part of the circle (-ARC-) in revolutionary (= anagram indicator) China (China = AN-HIC) (ANARCHIC),
2 Beef isn’t (beef isn’t = BENEFITS) bad (= anagram indicator) with a number of conveniences (BENEFITS),
3 Say hello (-HI-) to redhead (‘red’ head = -R-) in street (S-T) (SHIRT) from Jersey (jersey = SHIRT),
5 A foreign (a = UN-) fitness (-HEALTH-) centre (i) ? Radical (centre ‘Radical’ = -I-) and best (best) starting off (b) (‘best’ without ‘b’ = -EST) (UNHEALTHIEST) for the most unfit! (UNHEALTHIEST),
6 Nominates (nominates = MINNESOTA) doctor (= anagram indicator) on American border (MINNESOTA on US/Canadian border),
7 Navy (N-) is unsuccessful (failed), leaderless (f) (‘failed’ without ‘f’ = -AILED) (NAILED) and gets hammered (NAILED),
8 Visions (SIGHTS) of careers away from the cash registers (‘careers’ away from the ‘cash registers’ = shgist = SIGHTS),
11 Famous doctor with laid-back patients (psychoanalyst Sigmund FREUD-) linked to one (-I-) article (-AN) is at fault? (SLIP) (FREUDIAN SLIP) That said, it’s an unintentional error! (FREUDIAN SLIP),
15 Figures in a hotel (hotel rates = R-ATES) admit Oscar (-O-) has nothing (-O-) to do with Frenchmen (Messieurs = -MM-) (ROOMMATES) living together in the same accommodation (ROOMMATES),
16 Give a false picture (DISGUISE) to worried (= anagram indicator) guides (guides = D-GUISE) protecting One (-I-) Direction (direction south = -S-) (DISGUISE),
17 Sounds like advice to curb temper-tantrums (end anger = ENDANGER) is put at risk (ENDANGER),
19 Take a siesta (-NAP) after (= position indicator) minor (KID-) (KIDNAP) seizure (KIDNAP),
20 Short (LITTLE) mini (LITTLE),
23 Reviews (= reversal indicator) English (-E-) chapter of history (era = A-RE-) about key (music key -G-) (AGREE) match (AGREE).