Unidentified composer of piano nocturne (4)
I believe the answer is:
anon
'unidentified composer' is the definition.
'of piano nocturne' is the wordplay.
'of' says the answer is hidden in the clue.
'ANON' can be found hidden inside 'piano nocturne'.
(Other definitions for anon that I've seen before include "Incognito (abbrev)" , "Presently - or unknown" , "before long" , "Not identified by name" , "Soon, shortly (archaic)" .)