The secret language of fish -- English! (4)
I believe the answer is:
code
'the 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' acts as a link.
(Other definitions for code that I've seen before include "See 21 Across" , "Cypher" , "Kind of encryption designed to disguise content" , "Eg, Morse" , "difficult to read" .)