Turn once set to music (8)
I believe the answer is:
nocturne
'to music' is the definition.
(nocturne is a kind of music)
'turn once set' is the wordplay.
'set' is an anagram indicator (instruction to set the letters).
'turn'+'once'='turnonce'
'turnonce' anagrammed gives 'NOCTURNE'.
(Other definitions for nocturne that I've seen before include "Dreamy, pensive instrumental composition" , "Picture of a night scene" , "Dreamy instrument music" , "Dreamy piano peace popularised by Chopin" , "Chopin piece" .)