Arm muscles (6)
I believe the answer is:
biceps
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">arm muscles</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 biceps that I've seen before include "Weightlifters use them" , "Contractor" , "Arm muscles" , "Large upper arm muscle" , "One operating arm" .)