Across:
1 A quarrel (a rift = A-RIFT) over tip for Derby (tip for ‘Derby’ = -D-) (ADRIFT) off course (ADRIFT),
4 Considering (THINKING) most of those in 27 across (most of ‘thinks’ = THINK-) and in (-IN-) Government (-G) (THINKING),
9 Metropolitans dismiss trial on (‘Metropolitans’ dismiss ‘trial on’ = meopts = TEMPOS) playing speeds (TEMPOS),
10 Protective types (BULWARKS) of tree huggers perhaps, close to Hazel and Ash (barks = B-ARKS), welcome university (University of Limerick = -UL-) work (-W-) (BULWARKS),
12 Similar to Olive (olive green = LIME- green) and Pip (pip/seeds = -STONE) (LIMESTONE) from The Hard Rock (LIMESTONE),
13 Sounds like (= homophone indicator) something you’d get from The Treaty (get ‘peace’ = ‘PIECE’) and The Queen (PIECE on the chessboard),
14 It’s typical of an honest broker (SEEN TO BE FAIR) viewed as having the reputation of a blonde perhaps (SEEN TO BE FAIR),
18 Say what’s on your mind (THINK OUT LOUD) to very slender (THIN-) Kelvin (kelvin = unit of measurement = - K) on holiday (OUT) – it sounds harsh! (LOUD) (THINK OUT LOUD),
21 Yeats (Yeats = YEAST) mistaken (= anagram indicator) for Baker? (YEAST is for baker),
22 Team (SIDE) is soft (softly/piano = P-), never on time (-LATE) (SIDE PLATE), and is left holding the crumbs (SIDE PLATE),
24 That’s the last thing you might do (PASS AWAY) to sail through (PASS) a (A-) course (-WAY) (PASS AWAY),
25 Operatives lose vote (‘operatives’ lose ‘vote’ = praies = PRAISE) in The Laurels (PRAISE),
26 Genius (ARTISTRY) from the painting class (ART- class) is (-IS-) really appreciated by those playing rugby (-TRY) (ARTISTRY),
27 Uses one’s grey matter (THINKS) opening 18 across (‘think out loud’ = THINK-) at school (-S) (THINKS).
Down:
1 They’re painting a picture of model (ART CLASS) in part (part) initially lacking (p) (‘part’ without ‘p’= ART) elegance (CLASS) (ART CLASS),
2 Bear in mind (REMEMBER) one of the team (-MEMBER) is after (= position indicator) one of those flipping (= reversal indicator) Wards (one ward = er = RE-) (REMEMBER),
3 Courses (FLOWS) for raising (= reversal indicator) small (-S) type of animal (wolf = FLOW-) (FLOWS),
5 Accommodation (HOUSE) on various (= anagram indicator) floors (floors = OF LOR-S) holding key (music key -D-) (HOUSE OF LORDS) foreign peer group? (HOUSE OF LORDS),
6 Latest (NEW-) on 100 metres (-SPRINT) (NEWSPRINT) is in the paper (NEWSPRINT),
7 Earl is (earl is = ISRAEL) out (= anagram indicator) in the Middle East (ISRAEL),
8 Phrenologists missing, in short (‘phrenologists’ missing ‘in short’ = peolgs = GOSPEL), and that’s the absolute truth (GOSPEL truth),
11 Brush (COMB ONE’S HAIR) with company (CO-) doctor (-MB) by individuals (ones = ONE’S) leads to hostility and irrational rebuke (leads to ‘hostility and irrational rebuke’ = HAIR) (COMB ONE’S HAIR),
15 Bowler (hat = TAH-) knocks back (= reversal indicator) Asti in (asti in = -ITIANS) cocktail (= anagram indicator) (TAHITIANS) from South Pacific (TAHITIANS),
16 Spot (LOCATION) line (L-) by Oscar (-O-) from action (action = -CATION) novel (= anagram indicator) (LOCATION),
17 This has nothing to do (IDLENESS) with why someone can’t work out (IDLENESS),
19 Detective (-PI-) in Mayo (Mayo = MYO-A) confused (= anagram indicator) (MYOPIA) by lack of (lack of imagination/foresight = MYOPIA),
20 Hound (BASSET) produced by wild (= anagram indicator) beasts (beasts = BASSET),
23 Elite getting out of helicopter (‘elite’ getting out of ‘helicopter’ = hcopr = PORCH) providing space for the American rockers (providing space for the Americans on their rockers/rocking chairs = PORCH).