Insect then, or fishy (6)
I believe the answer is:
hornet
'insect' is the definition.
(hornet is a kind of insect)
'then or fishy' is the wordplay.
'fishy' indicates anagramming the letters.
'then'+'or'='thenor'
'thenor' anagrammed gives 'HORNET'.
(Other definitions for hornet that I've seen before include "Large wasp with vicious sting" , "Vespa" , "Stingy type" , "Stinging insect" , "A large wasp with severe sting (6)" .)