Fish with a tail (4)
I believe the answer is:
coda
'tail' is the definition.
(I've seen this before)
'fish with a' is the wordplay.
'fish' becomes 'cod' (type of fish).
'with' means one lot of letters go next to another.
'cod'+'a'='CODA'
(Other definitions for coda that I've seen before include "Concluding passage of piece of music" , "Closing bars (mus.)" , "Passage concluding a musical piece" , "End piece written for a musical work" , "Concluding passage of a musical composition" .)