Performance in once jazzy musical composition (8)
I believe the answer is:
nocturne
'musical composition' is the definition.
(nocturne is a kind of musical composition)
'performance in once jazzy' is the wordplay.
'performance' becomes 'turn' (turn is a kind of performance).
'in' is an insertion indicator.
'jazzy' is an anagram indicator.
'once' anagrammed gives 'noce'.
'turn' going within 'noce' is 'NOCTURNE'.
(Other definitions for nocturne that I've seen before include "Dreamy musical piece" , "Picture of a night scene" , "Dreamy instrument music" , "Musical item" , "Dreamy, pensive instrumental composition" .)