The unclued lights (two of two words) can be all be preceded by the same word which solvers have to highlight appropriately in the completed grid. All are confirmed in Brewer apart from one, which is not listed in Chambers either (surprisingly).
 

Across

1    Spanish mansion with some basic fruit (7)
6    What IoW tourists may do, it’s said, to manatees (7, two words)
11    Sheath’s brownish-yellow area (6)
12   Protective garments we put on, releasing right arms (7)
14    Some jubilee arranged for Glaswegian porkpie merchant (5)
17    More than one giant bird — bit like a goose (6)
19    Large band putting odd Scottish item of furniture inside (9)
21    Now seen about in Ascot and Bow? They could be (7)
24    Metamorphosing engineered without porno US headwear (9, two words)
26    Holding a map, observes aircraft (9)
29    Old woman without love runs to Bing, say (7)
30    Fool catches Father Henry with unknown spontaneous illness (9)
35    Bone from ancient city (5)
36    Song of praise and peace wasn’t regularly found (5)
39    Hollow beside River Plate (7)
40    Came out at end of soiree and mingled (7)
 

Down

1    Pamphleteer and MP at lowly building with Fritz’s bed (7)
2    Novelist reports photographic session (5)
3    Old award from some literary upset (4)
5    Promises sport aboard ship (6)
6    Cashmere weave? I’ve no idea! (8, two words)
7    Cockney houses and burrows (6)
9    Nostradamus unusually avoiding sum of musquashes (8)
10    Looks around, saying the kids will play on these (7)
13    Astragalus from crazed weakling (8)
18    DDR once put in trouble — with this result? (10, two words)
20    Eastern market finally ordered coarse fabrics (8)
22    North American native giving note to candidate (9)
23    Deciphered Arabic in referring to ancient seafaring gods (8)
25    One egg I consumed, to start (8)
26    Frustrated at home reportedly, that is in a state (7)
27    Nurtured artist in recording was dominant (7)
28    Cheat loses his head for musician (6)
31    Psychiatrist at home during delay (5)
 
A first prize of £30 for the first correct solution opened on 4 June. There are two runners-up prizes of £20. (UK solvers can choose to receive the latest edition of the Chambers dictionary instead of cash — ring the word ‘dictionary’.) Entries to: Crossword 2359, The Spectator, 22 Old Queen Street, London SW1H 9HP. Please allow six weeks for prize delivery.