Marx's co-author (6)
I believe the answer is:
engels
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">marx's co-author</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)