The tail part of a fish, note (4)
I believe the answer is:
coda
'the tail' is the definition.
(I've seen this before)
'a fish note' is the wordplay.
'a fish' becomes 'cod' (type of fish).
'note' becomes 'a' (musical note).
'cod'+'a'='CODA'
'part of' is the link.
(Other definitions for coda that I've seen before include "Passage concluding a musical piece" , "noted conclusion" , "tail" , "End piece written for a piece of music" , "Novel ending" .)