Of an arm bone (5)
I believe the answer is:
ulnar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">of an arm bone</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 ulnar that I've seen before include "of certain bones" , "Relating to a bone in the forearm" , "Relating to one of the bones of the forearm" , "Relating to a particular arm bone" .)