Tailpiece - a fish has one (4)
I believe the answer is:
coda
'tailpiece' is the definition.
(I've seen this before)
'a fish has one' is the wordplay.
'a fish' becomes 'cod' (type of fish).
'has' means one lot of letters go next to another.
'one' becomes 'a' (a thing is one thing).
'cod'+'a'='CODA'
(Other definitions for coda that I've seen before include "Tailpiece" , "Word that modifies" , "Final section" , "Musical conclusion" , "Final part of mus. structure" .)