Kind of monkey
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">kind of monkey</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for rhesus that I've seen before include "Type of macaque" , "Usually a positive factor in man" , "Type of monkey and blood category" , "King, in Greek mythology" , "Kind of monkey or blood group" .)