Fishy letter not in plain English (4)
I believe the answer is:
code
'not in plain english' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'fishy letter' is the wordplay.
'fishy' becomes 'cod' (type of fish).
'letter' becomes 'e' ().
'cod'+'e'='CODE'
Can you help me to learn more?
(Other definitions for code that I've seen before include "Method of encryption or set of rules" , "ethical principles?" , "Playfair is one" , "Computing instructions" , "Secret language" .)