Across:
1 Introduces 18 across (introduces 'keep a promise' = KEEP-) to East (-E-) writing (from the 3Rs = -R) (KEEPER) for The Guardian (KEEPER),
4 College (-C-) in Ireland (.ie = I-E-) provides accommodation (-HOUSE) (ICEHOUSE) in a cold place surrounded by a number of blocks (ICEHOUSE),
9 Middlemen (middle ‘mEn’ = E-) dead (-LATE-) tired at the foundation (‘tireD’ at the foundation = -D) (ELATED) or in high spirits? (ELATED),
10 People tend to open up for them (DENTISTS), as they are capable of building bridges (DENTISTS),
12 Misfit (= anagram indicator) learns (learns = SLAN-ER) about the foreign (‘the’ foreign = -DER-) (SLANDERER) mudslinger (SLANDERER),
13 Dudley Moore releases medley (‘Dudley Moore’ releases ‘medley’ = udoor = ODOUR) with an unpleasant air (ODOUR),
14 Refuse to be baited (KEEP ONE’S HEAD) by individual (ONE -) in 21 across (‘keeps’ = KEEP -S) and The Crown (HEAD) (KEEP ONE’S HEAD),
18 Don’t renege on guarantee (KEEP A PROMISE) to American (A) after (= position indicator) most of those in 21 across (most of ‘keeps’ = KEEP) take oath (PROMISE) (KEEP A PROMISE),
21 The Castle’s (the castles = KEEPS) reserves (KEEPS),
22 Return the compliment (RETALIATE) in a literate (a literate) novel (= anagram indicator) (a literate = RETALIATE),
24 As it happens (-LIVE), Oscar (O-) is in Greece (Greece) reporting (= homophone indicator) (‘Greece’ = ‘grease’ = oil = O- -IL) (OLIVE OIL) to a crude type that’s always pressed (OLIVE OIL),
25 Serves up (= reverse indicator) fish (hake) without head (h) (‘hake’ without ‘h’ = ake = -EKA) from Russia (Ru = -UR-) and the point (East = E-) (EUREKA)? I found it! (EUREKA),
26 Pine (pine tree = TRE-E) over a (-A-) model (model = sit) recalled (= reverse indicator) (sit = -TIS-) (TREATISE) in a written study (TREATISE),
27 Crafty type (boat/ship = -SS-) in River (River Lee = LE-E-) Erne – the first (‘Erne’ the first = -E) (LESSEE) one paying a flat fee perhaps? (LESSEE).
Down:
1 Lay down (KEEP-) type of wine (-SAKE rice wine) (KEEPSAKE) for the memorial (KEEPSAKE),
2 Upset (= reverse indicator) heartless Greek character (‘delta’ without ‘l’ = deta = -ATED) and celebrity (name = EMAN-) (EMANATED) gave out (EMANATED),
3 Some of the Orangemen deceived (some of the ‘orangEMEN Deceived’ = EMEND) by change (change/alter = EMEND)
5 Make way (CLEAR THE PATH) for Shakespearean character (-LEAR) in court (ct = C- T-) – that man (-HE) passing around the hat for tips (‘Passing Around The Hat’ for tips = PATH) (CLEAR THE PATH),
6 Good books (Old Testament = -OT-) associated with Kitty (POT-) in Panama (panama hat = H- -AT-) ahead of opposition (ahead of ‘Opposition’ = -O) (HOT POTATO) to controversial issue (HOT POTATO),
7 Outcome (UPSHOT) of a wake (‘a wake’ = ‘awake’ = UP-) for some leader (‘Some’ leader = -S-) at the start of 6 down (start of ‘hot potato’ = -HOT) (UPSHOT),
8 Guard (guard/secure = ENSURE) loses the head (c) with official rebuke (censure) (‘censure’ without ‘c’ = ENSURE),
11 Political groups saving energy (GREEN PARTIES) for celebrations outside the clubhouse with the swingers (golfers) (GREEN PARTIES),
15 Out all night, more so than usual (OVERSLEPT) – it’s what she did awaiting the handsome prince? (Sleeping Beauty OVERSLEEP),
16 Different (= anagram indicator) dialects (dialects = CITADELS) in the Salvation Army meeting places (CITADELS),
17 What you might get to drink (BEVERAGE) at all times (-EVER-) from head barman (Attorney General = -AG-) in Belgium (BE = B-E) (BEVERAGE),
19 Scrape by (EKE OUT) with energy (E-) cocktail (‘cocK’ tail = -K-) from Spain (-E) no longer in fashion (not in = OUT) (EKE OUT),
20 Right (R-) time (-T-) in Ireland (Eire = -E-IRE) (RETIRE) to give up working (RETIRE),
23 Local unpleasant person (LOUSE) running (= anagram indicator) Seoul (Seoul = LOUSE).