Four of the unclued lights (one of two words) and an unclued pair are of a kind, and can become the remaining unclued lights, including a pair.
 

Across

 
1    This song of praise would become heathen with a change of note (5)
6    Failure of old skinflint at university (7, hyphened)
11    Catlike print he has developed (10)
17    Tramp’s symbol (7)
18    Tolkien creature at hospital? (3)
19    Perform rapid chords, rearranging gig with a repeat (10)
21    Tentative effort producing sort of chain letter — always (5)
24    Star sign regularly appearing for triplet (4)
30    From which to serve and yet come back round to consume bread (10, hyphened)
33    French art embracing new existence (3)
35    Catching poor orphan, finally, in rags (7)
36    Error, when duke leaves, causes bafflement (7)
37    Opposed to chartered surveyor leaving 10 (4)
39    Agitated Frenchman coming in after 9, drunk (10, three words)
 

Down

 
1    Imaginary feeling turning up as the disease (14)
2    Tension about seedings (8)
3    Calm old gent’s first seen in avenue (6)
5    Some unconditional rumour (5, hyphened)
6    Settlers cooking fish (8)
8    Prince’s commands (5)
9    Those girls dancing with OAPs, they love their expressions (14)
10    Old buffoons frequently left wild society (6)
12    Annoyed with two and a half dozen abandoning Thailand for Sweden (6)
14    Price includes fashionable fabric (6)
22    Manifest when watching the Ashes? (6)
23    Skin tumour making male moan (8)
25    Take one’s life when girl turns tail. Remarkable! (8, three words)
26    Bird’s tailless fish (6)
29    Cover for an annexe, about noon? (6)
31    Water buffalo consuming one cloudy tincture (6)
32    Political prison hoax about mid-July (5)
35    Ancient civilization upset founder of another (5)
 
A first prize of £30 for the first correct solution opened on 27 April. 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 2206, The Spectator, 22 Old Queen Street, London SW1H 9HP. Please allow six weeks for prize delivery.