Secret language of fish — English? (4)
I believe the answer is:
code
'secret language' is the definition.
(thesaurus)
'fish english?' is the wordplay.
'fish' becomes 'cod' (cod is a kind of fish**).
'english?' becomes 'e' (abbreviation).
'cod'+'e'='CODE'
'of' is the link.
(Other definitions for code that I've seen before include "Eg, Morse" , "Cipher - set of principles" , "what's character-changing?" , "Cypher" , "See 12" .)