Fish devouring no-good political theorist (6)
I believe the answer is:
engels
'political theorist' is the definition.
(philosopher Friedrich Engels)
'fish devouring no-good' is the wordplay.
'fish' becomes 'eels' (I've seen this before).
'devouring' is an insertion indicator.
'no' becomes 'n' (common abbreviation).
'good' becomes 'g' (abbreviation).
'n'+'g'='ng'
'eels' going around 'ng' is 'ENGELS'.
(Other definitions for engels that I've seen before include "German philosopher and collaborator of Marx" , "Early socialist" , "Marx's patron" , "Friedrich --, associate of Karl Marx" , "German political philosopher who collaborated with Marx" .)