"The Communist Manifesto” co-author (6)
I believe the answer is:
engels
'the communist manifesto co-author' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for engels that I've seen before include "Karl Marx's collaborator" , "Early socialist" , "Friedrich --, associate of Karl Marx" , "German philosopher and collaborator of Marx" , "Friedrich -, Marx collaborator" .)