Across:
1 Board member (chess board KNIGHT) to consider (think) retiring (= reversal indicator) (think = KNI-HT) around end of spring (end of 'sprinG' = -G-) (KNIGHT),
4 Gardener (BEDMAKER) to the queen’s architect perhaps? (queen bed’s architect = BEDMAKER),
9 Sings the praises (EXTOLS) of former partner (EX-) lost (lost) at sea (= anagram indicator) (lost = -TOLS) (EXTOLS),
10 Must (must) rearrange (= anagram indicator) (must = STUM-) half the accommodation (half of ‘B&B’ = -B-) for the French (the = -LES) (STUMBLES) trips (trips/falls = STUMBLES),
12 Observe two of everything (SEE DOUBLE) in Mediterranean (sea) State (= homophone indicator) (‘sea’ = ‘SEE’) twice the usual size (DOUBLE) (SEE DOUBLE),
13 Part of the orchestra (wind instruments = REEDS) turns up (= reversal indicator) sound from the start (‘Sound’ from the start = -S) on stag (deer = REED-) (REEDS),
14 1 across (‘knight’ = KNIGHT) and 20 down (‘errant’ = ERRANT) (KNIGHT ERRANT) from Don Quixote, for one (KNIGHT ERRANT),
18 It reduces the tension in the lab (WETTING AGENT) by pouring hot water, for instance (WETTING), on most of those in 27 across (most in ‘agents’= AGENT)! (WETTING AGENT),
21 Loses the head (p) on drink (‘port’ without ‘p’ = -ORT-) and goes into Alcoholics Anonymous (A-A) (AORTA) – will the cardiologist be able to handle it? (AORTA).
22 Cautioned (cautioned) playing (= anagram indicator) (cautioned = EDUCATION) in the classy practice for those in the academy (EDUCATION),
24 Local fruitcake (‘local’ fruitcake = HEADCASE) from Schull (Schull) by the sound of it perhaps (= homophone indicator) (‘Schull’ = ‘skull’ = head case = HEADCASE),
25 Frets leaving shoplifters (‘Frets’ leaving ‘shoplifters’ = shopli = POLISH) to those in the community (those in the European community = POLISH),
26 Trip (outing = OUT-ING) around heartless (a) place (lay) (‘lay’ without ‘a’ = -LY-) (OUTLYING) that’s remote (OUTLYING),
27 Gladstone dismissed old (‘Gladstone’ dismissed ‘old’ = gastne = AGENTS) spies (AGENTS).
Down:
1 Lay down (KEEP-) type of wine (-SAKE rice wine) (KEEPSAKE) for the memorial (KEEPSAKE),
2 One (I-) Pinter (Pinter) play (= anagram indicator) (Pinter = -NTREPI-) directed (-D) (INTREPID) by fearless type (INTREPID),
3 Often the first thing out of characters’ mouths (HELLO) in The Inferno (HELL-) is to do with love (-O) (HELLO).
5 The risk-taking, moneymaking producer (ENTREPRENEUR) of The Merchant of Venice, for instance (ENTREPRENEUR),
6 It’s the story of one’s life (memoir) – missing one (i) (‘memoIr’ without ‘i’ = MEMOR-) as well as (-AND-) a (-A) (MEMORANDA) series of notes providing reminders (MEMORANDA),
7 Shrink from foundation (‘shrinK’ from foundation = K-) sick (-ILL-) in hospital (-ER) (KILLER) – to the locals it’s a joke (locally a ‘joke’ = KILLER),
8 Don’t be taken in by the arguments (RESIST) to join the Freedom Fighters (fight = RESIST),
11 Take issue (OBJECT) with Takeaway (take away = LESS-) with no (no) backing (= reversal indicator) (no = -ON) (OBJECT LESSON) as an instructive experience, for example (OBJECT LESSON),
15 Brief (t) success (hit) (‘hit’ without ‘t’ = HI-) at archery (archery) confusing (= anagram indicator) (archery = -ERARCHY) (HIERARCHY) top people in organisation (HIERARCHY),
16 Judge’s sentence (DECISION) remarkably (= anagram indicator) is in code (is in code = DECISION),
17 Stinks (STENCHES) of small (S-) fish (-TENCH-) starters with edible snails (starters with ‘Edible Snails’ = -ES) (STENCHES),
19 Pressure (P-) to find proof of a fire (-ASH-) around (= reverse indicator) good books (Old Testament = OT = -TO) (PASHTO) – you hear this all the time in Afghanistan (PASHTO),
20 A lot fell off alternator (‘a lot’ fell off ‘alternator’ = ternar = ERRANT) and it’s not in the right place of course (ERRANT),
23 In the middle of (AMONG) mango (mango) cocktail (= anagram indicator) (mango = AMONG).