Swimmer with a tailpiece (4)
I believe the answer is:
coda
'tailpiece' is the definition.
(thesaurus)
'swimmer with a' is the wordplay.
'swimmer' becomes 'cod' (cod is a kind of fish**).
'with' says to put letters next to each other.
'cod'+'a'='CODA'
(Other definitions for coda that I've seen before include "Ending of musical piece" , "Passage of piece of music added to the end" , "Tailpiece" , "last piece of work" , "Closing bars (mus.)" .)