Fish has a tailpiece (4)
I believe the answer is:
coda
'tailpiece' is the definition.
(I've seen this before)
'fish has a' is the wordplay.
'fish' becomes 'cod' (type of fish).
'has' says to put letters next to each other.
'cod'+'a'='CODA'
(Other definitions for coda that I've seen before include "Musical finale" , "Closing passage of musical piece" , "Tailpiece" , "Final part of mus. structure" , "Closing stage" .)