There’s rather a pleasant spread of general knowledge in this one in the sense that you probably have most of it knocking around not too far from the surface: places, people, flora, fauna, astronomy and medical stuff all being well below the level expected of (say) University Challenge, and in general, the wordplay being kind enough if you have to scrape around for the answers. I trundled through in 21.38, with 17 down resisting to the end only because I misread the first word of the clue as “any”. All in all, a calming antidote to the frenetic politics of the day, though 19 conjured up unbidden that extraordinary shot of the Tangerine One presiding over a table laden with fast food goodies on silver platters, presumably all stone cold by the time the victorious Clemson University Tigers got their teeth into them. We live in weird times.
3 across is unusual in that it has two perfectly good answers not resolved by the wordplay. Otherwise, I present my findings as usual with clues, definitions and SOLUTIONS[Click for enlightenment]
1 Vow of all trusty henchmen at the start (4)
OATH So we start off with a take the first letters clue, Of All Trusty Henchman
3 Mediterranean islander is swopping halves for stars (5,5)
CANIS MAJOR Take your Med islander, in this case a MAJORCAN, add the IS, split down he middle and swop over the two halves. No particular reason why it shouldn’t be the Minor version until 7d resolves the ambiguity.
9 Chap in Scottish resort’s spoken of postal system (7)
AIR MAIL The resort is Ayr, the chap therein is male, speak them out loud and ink in the result
11 Traveller to Hebridean islands crossing river (7)
TOURIST It’s TO (given in plain view) UIST (there are two of them, North And South, hence the plural) with R(iver) inserted. Think it works.
12 Pipes, say, relaid after landlord’s latest complaint (9)
DYSPEPSIA An anagram (relaid) of PIPES SAY tacked on to the last letter of landlorD
13 Animal that's long occupying heart of Sahara (5)
HYENA Well, it doesn’t actually occupy the Sahara, but you need the middle two letters (heart) of saHAra and YEN for long to be inserted
14 Meadow and fen almost completely rank (5,7)
FIELD MARSHAL Meadow supplies FIELD, fen MARSH and completely (almost) supplies AL(l)
18 Record view about contest with you once (7-5)
SEVENTY EIGHT Before vinyl span at 33⅓ or 45, shellac whizzed round at 78 rpm, challenging steel needles to extract recorded sound. Here our construction is view: SIGHT around contest: EVENT and YE representing you, as the clue says, once.
21 Far East city's very big area including a ski centre (5)
OSAKA Very big (OutSize) A(rea) containing the centre of sKi
22 Job-holder has to copy across fifth suggestion? (9)
APPOINTEE Clever this. Copy is obviously APE, the rest is provided by imagining the fifth suggestion would be POINT E
24 By end of round, Rocky's knocked out (7)
DRUGGED End of round, plus RUGGED for Rocky ignoring the capitalisation
25 Get ready to fence section of kitchen garden (2,5)
EN GARDE Today’s hidden, fairly transparently, in kitchEN GARDEn. Perhaps helped by the split staying the same
26 Current number one in golf, masterful in play (4,6)
GULF STREAM A long-winded indication for G from Golf, plus an anagram (in play) of MASTERFUL
27 Old secretary left item in Hatton Garden? (4)
OPAL Hatton Garden is both a street and area synonymous with the jewellery (especially diamond) trade, so might contain opals, O(ld) PA (secretary) L(eft)
1 Cricket sides not consistent? (2-3-3)
ON-AND-OFF Effectively two definitions, though the first leads to an unhyphenated version
2 During the day, hives maybe given whitewashing (8)
THRASHED Hives here not the bee residences, but nettle RASH and other itchy inflictions. THE D(ay) doesn’t ask you for a specific, just to wrap the letters around the RASH
4 Range in the end lacking finish (5)
ATLAS In the end is AT LAST. Knock off the end for the mountain range opposite Gibraltar (and quite a lot east and west)
5 Travelling home, time flew: it’s strange! (2,7)
IN TRANSIT Home provides IN, T(ime) continues, flew gives RAN, and a strange version of IT’S gives SIT
6 Very tempting argument and how it developed (13)
MOUTHWATERING The anagram fodder to be developed is ARGUMENT + HOW IT. Took me a while (and some helpful crossers) to organise.
7 Saw user maybe as clubbable type? (6)
JOINER A double definition the first from a tool of his trade
8 Furniture-maker's revolutionary new aromatic oil (6)
RATTAN Not this time the person, but the material used derived from climbing palms of the same name seen in varieties of wickerwork. We derive it from N(ew) ATTAR (as in of roses) combine and reversed (revolutionary)
10 End user began originally to go after a cattle breed (8,5)
ABERDEEN ANGUS Took me until this point to see that it’s no more than an anagram of END USER BEGAN tacked on to an initial A
15 A Greek swimmer brings in wrong sign for czar (9)
ALEXANDER LEANDER nightly swam across the Hellespont (a tricky minimum of 1.2 kilometres) for love of the fair Hero. Add the A, insert an X (sign for wrong) and you end up with one of two Czars
16 Politically loaded material has a good supporter penning it (8)
AGITPROP A G(ood) supporter: PROP surrounds IT
17 Airy number with details every so often (8)
ETHEREAL Number in the sense of something that makes numb produces ETHER, to which you add the even letters of dEtAiL
19 Greyhound stadium's favourite snack (3,3)
HOT DOG I think the clue splits as indicated, with the (red hot) favourite greyhound being the HOT DOG. That way you avoid arguments about hot dogs being more favoured than (say) salmonella burgers, pies or prawn sandwiches.
20 Offhand and cold as usual, ignoring our lot (6)
CASUAL C(old) plus As USUAL, disposing of US: our lot
23 Mary's portrait of two Athenian characters (5)
PIETA A pieta is a depiction of Mary cradling her deceased son: we need Π and Η (that’s pi and eta to you) to construct our version.