Marx's co-author (6)
I believe the answer is:
engels
'marx's co-author' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for engels that I've seen before include ". . . and his friend, Friedrich" , "Communism founder" , "Early Communist philosopher" , "Communism's co-founder with Marx" , "He co-authored The Communist Manifesto with Karl Marx" .)