Unclued lights (including two of two words and an accent to be ignored) can be expressed in such a way as to form a thematic set. Elsewhere, ignore an apostrophe.
 

Across

 
1    Solitary criminal canned opium (11)
11    Vintage malt liquor needing special can in perfect condition (6)
13    Dramatist from Italy a certain ancient poet abridged (7)
15    Knockout pair of rivers divided country (5)
16    More than one duck appears in turn (5)
17    Implant in Henry’s heart and back as well (6)
18    What’s by Mac’s door? Rook heartless cat carried round (5)
20    Negative voter puts out tent for Wesley? (6, hyphened)
21    Low-down on a type of cake (5)
22    Pessimistic precept or maxim grabbing sorry old square (7, two words)
27    Drummer not so good: he’s boring (7)
29    Bone crosses with Latin inscribed (5)
30    East Indies market vinegar no longer (6)
34    Tell what this is – state briefly (6, two words)
36    The reverse of chicken, game old statesman (5)
37    Extraterrestrial Buzz A’s commander brought back (5)
38    Organic coats amateur lines eventually (7)
39    Move on having lived years with relative (6)
40    Pop is bashful (3)
 

Down

 
2    Hundreds are unable to clear up ultimate in messes (8)
3    Sea ducks endure endless tall boats (9)
5    Horrific place for nine pounds (7)
6    I’m often pressed to meditate about successfully giving up whiskey (8)
7    Tea some buyer bags? (5)
8    Pure bull! (4)
9    Driven round island … (6)
10    … criminal met missing lecturer (5)
14    Flash tie is alternative choice (12, two words)
21    Look into sinfulness of deadly inventor (9)
23    Piece of machinery to clean out fish boxes (8, two words)
24    Fly servant becomes head of guild (8)
25    China the Spanish enthralled with a stew (6)
26    Cardinal houses goddess in city (7)
28    Independent judge blocks return of robust prophet (6)
31    Plant completely drowned by waterfall (5)
33    Disaster when Sunshine State withdraws friendship (5)
 
A first prize of £30 for the first correct solution opened on 24 February. There are two runners-up prizes of £20 (or, for UK solvers, the latest edition of The Chambers Dictionary — ring the word ‘Dictionary’). Entries to: Crossword 2148, The Spectator, 22 Old Queen Street, London SW1H 9HP. Please allow six weeks for prize delivery.