Bone of the arm (4)
I believe the answer is:
ulna
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">bone of the arm</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for ulna that I've seen before include "a section of framework" , "The longer and inner bone in forearm" , "Thinner and longer of two bones in forearm" , "structural member?" , "Long bone in the arm" .)