End of fish, end of sea (4)
I believe the answer is:
coda
'end' is the definition.
(thesaurus)
'fish end of sea' is the wordplay.
'fish' becomes 'cod' (cod is a kind of fish**).
'end of' suggests the final letters.
The last letter of 'sea' is 'a'.
'cod'+'a'='CODA'
'of' acts as a link.
(Other definitions for coda that I've seen before include "Conclusion of work" , "Final section" , "noted conclusion" , "finale in Beethoven?" , "notes at the end" .)