Pest in male fish (9)
I believe the answer is:
cockroach
'pest in male fish' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'male fish' is the wordplay.
'male' becomes 'cock' (male chicken).
'fish' becomes 'roach' (roach is a kind of fish).
'cock'+'roach'='COCKROACH'
Can you help me to learn more?
(Other definitions for cockroach that I've seen before include "Cork coach can be a pest" , "Household insect pest" , "See pest" , "So-called black beetle" , "Rock coach turns out to be an insect" .)