The unclued lights (two of two words) be paired and are linked by an anagram of the four letters in the yellow squares. Brewer verifies the theme.
 

Across

1    No stamp confused Pat. Could be him! (7)
6    Optical expert’s endless worry returning item for camera (7)
12    Gave notice of danger of now withdrawn chemical by the sea (7)
14    Disturbances in Stroud not so regularly (5, hyphened)
15    Little men on the front row (5)
16    Bad criminal having time in ancient Greek city (6)
17   Way to anticipate enormous sadness (6)
19    Could be Orange where slavery did not exist (9, two words)
21    Moths or some pachyderm in Essex (7)
24    Silk and crepe ordered for young swimmers (9)
26    Wimbledon favourite’s purse forming capital to set up a project (9, two words)
30    Airless arena demolished with old boy in charge (9)
32    The last rounds when decorative toppings lose one (6)
33    Sounds of personal transport in Cornish mines (6)
35    Trifle endlessly — in a way (5)
36    Completely ruin tenor and alto work (5)
37    Majority for Brexit result, apparently? (7)
38    Harris’s jacket (6)
39    Fruit from mother and children (7)
40    Very thick-headed son in study-group (7)
 

Down

1    Petition — but for Caesar would be satisfied (7)
2    Fletcher’s prison group … (5)
3    … the right moment for porridge (4)
4    It provides the beat from the underground department (9)
5    Low emotional conditions affected noon raids (6)
7    Senior seaman went fast and spoke with a rough voice (6)
8    Just a little square — being of consequence (10)
10    Belittles the French city (7)
20    Revealing injunction to liar? (8)
22    Field events last month at Irish city cut short (9)
23    House-warming show left out faded flower (8)
25    Popular couple of men with many smudges (8, two words)
26    With workers appointed and stopped replacing learners with females (7)
27    Fool, perhaps, locked up? (7)
29    Showing skill when presented with a simple goal (6)
31    PM extremely believable? Possibly (5)
 
A first prize of £30 for the first correct solution opened on 11 November. 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 2431, The Spectator, 22 Old Queen Street, London SW1H 9HP. Please allow six weeks for prize delivery.