Nine unclued lights are linked in three different ways (three lights to each) to a tenth, which solvers should highlight. All may be verified in Chambers.
 

Across

 
5    Wet month unproductive (6)
10    In the open, fish got cleverly hidden (10, three words)
12    Atom bombs are nuclear without exception (6, three words)
17    Trifled foolishly? (7)
18    Request provided for a time obliquely in Scotland (7)
20    In part of swine’s face, note a drop of white (8)
25    Look round Spain for a house (3)
26    Prelates leaving a cathedral en masse? (7)
28    Second petition secures a red dress (7, hyphened)
29    Say Gabon is the source of chicken (or is it?) (3)
31    Instrument for copying two letters sounding like one opened up by amateur (8)
34    Impress soldiers into military machine shortly (7)
36    In earliest times, man penetrated endless mysteries (7)
40    Everything purchased outright?  I may come in, primed with the details (8)
41    One finishing school that can be used to gain advantage, say (6)
42    Foolishly insulated a mineral (10)
 

Down

 
1    Strike keeps telegraph office unable to communicate (6, two words)
2    Vaguely heard recipe:  funny, no starter (8)
3    With spine broken, nothing they can carry? (6)
4    A hospital has a position for meditation (5)
6    A cloth put over jelly (4)
7    Annoyed having tons to hold, ring man in depot (11)
8    Once convenient hour to finish (4)
11    Having double alcohol, eat less, extremely happy at end of meal (7)
14    What is the point of the moon?  Copper and silver production, originally (4)
15    Orderly is really to be executed (4)
16    Sort of language union faring badly (11, hyphened)
21    Ready in Lesotho for start of auction? (4)
22    Not entirely wide awake, is the impression (4)
23    Be in headgear under a peak — one learns this early (8)
27    He hates a straggly pink flower (8, two words)
30    Frank indecent? That’s not literal (4)
31    Poem about end of peace conference (4)
32    Smile briefly on college swimmer (6)
33    Australian alas missing getting excited here? (6, three words)
35    Woman’s book not very entertaining at first (5)
37    Fish following through tangle (4)
38    Ruin part of Nessun Dorma (4)
 
A first prize of £30 for the first correct solution opened on 12 May. 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 2159, The Spectator, 22 Old Queen Street, London SW1H 9HP. Please allow six weeks for prize delivery.