Radius partner (4)
I believe the answer is:
ulna
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">radius partner</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for ulna that I've seen before include "The longer and inner bone in forearm" , "Thinner of two long bones in forearm" , "Part of bone-structure" , "a section of framework" , "One in arms" .)