One unclued light (four words) is the title of a 40 recorded by 43 (two words). This title forms a cryptic indication of one unclued light, which defines each of the other unclued lights.
 

Across

11    Unlucky guys taken in by bad old lie (9, hyphened)
12    Try to turn round cubic mineral (4)
17    Noted folio poorly made (5)
20    Proceed, worried after revolutionary call (7)
21    Favouring relations, quickly write back (7)
24    Tea in family’s village in Scotland (7)
25    Journal from Suez in Egypt (5, hyphened)
26    Old letter with very large moral significance (5)
28    Mass consisting of protein (7)
31    Great batsman embracing drama hangs around (7)
34    Hare continually inhibited by obstruction (7)
38    Stop and cheer dance (5)
39    Stay seated longer than silly tourist ignoring rule (6)
41    Vulture, cross after festivity, holding nothing back (9)
42    Gazelle’s second hour (4)
 

Down

1    Figure is rising in vain (6)
2    Trees forming boundary (5)
3    In command, in retreat? Most delicate (6)
4    Artist upset about current series of woes (5)
5    Things on a menu ordered (7)
6    Man saw subsidy paid to farmers (7)
7    Impression about point of partition (6)
8    Made smart demand to accommodate a cardinal (8)
10    Insects round billet, see, all over the place (10, two words)
13    Notice ambassador after trade without translator (7)
22    So Rodin worked under cover? (7)
23    Grain pest nearly missing section protected by raised border (8, two words)
27    Staff officer blithely discharging me in contemptible way (7)
29    Thoroughly good joke surpassed (7)
30    Glorification from sailor after narcotic drink (6)
32    Remain on foot for convenience (6)
33    Odds at this point in range (6)
35    Country right above river (5)
36    Element in slang lacking justice (5)
 
A first prize of £30 for the first correct solution opened on 31 July. 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 2318, The Spectator, 22 Old Queen Street, London SW1H 9HP. Please allow six weeks for prize delivery.