Fish with a tailpiece (4)
I believe the answer is:
coda
'tailpiece' is the definition.
(thesaurus)
'fish with a' is the wordplay.
'fish' becomes 'cod' (cod is a kind of fish**).
'with' is a charade indicator (letters next to each other).
'cod'+'a'='CODA'
(Other definitions for coda that I've seen before include "Closing section of a musical composition" , "Musical recap" , "Cocluding musical passage" , "End piece written for a musical work" , "Musical finale" .)