Arthur Marx
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">arthur marx</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for harpo that I've seen before include "brother who never speaks" , "silent film star" , "One of the Marx Brothers" , "speechless brother" , "The mute Marx Brother" .)