Unidentified composer of piano nocturne (4)
I believe the answer is:
Here is my best explanation:
'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 "Having no known name", "immediately", "soon to be written", "Author unknown", "Some time soon", "Soon; unnamed", "Soon, precious stone", "coming after a call", "Soon - nobody knows me" and "Soon - nameless".)