Marx's collaborator on "The Communist Manifesto"
I believe the answer is:
engels
'marx's collaborator on the communist manifesto' 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 "political theorist" , "Early socialist" , "Friedrich .. . . . . founded communism with Karl Marx" , "Marx's co-author" , "Friedrich --, associate of Karl Marx" .)