Secret language of fish eaters, primarily (4)
I believe the answer is:
code
'secret language' is the definition.
(I've seen this in another clue)
'fish eaters primarily' is the wordplay.
'fish' becomes 'cod' (type of fish).
'primarily' says to take the initial letters.
The first letter of 'eaters' is 'e'.
'cod'+'e'='CODE'
'of' acts as a link.
(Other definitions for code that I've seen before include "(Computer) programming instructions" , "System of rules" , "Cypher" , "Cryptographic system" , "ethical principles?" .)