Biceps or triceps (6)
I believe the answer is:
muscle
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">biceps</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">triceps</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ ></span>
(Other definitions for muscle that I've seen before include "Bicep or tricep, for instance" , "contractor" , "Physical power" , "Beef" , "Part of the body which produces movement" .)