Marx co-author (6)
I believe the answer is:
engels
'marx 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 "German linked to Marx" , "Close collaborator of Karl Marx" , "Communism's co-founder with Marx" , "Friedrich .. . . . . founded communism with Karl Marx" , "Friedrich --, associate of Karl Marx" .)