Arm bones (5)
I believe the answer is:
radii
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">arm bones</span>' is the definition.<br/ ><span class="explanation-format__explanation">(radius is a kind of arm bone)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for radii that I've seen before include "Halves of diameter" , "parts for arms" , "Straight lines from the centre to the surface of a sphere" , "Spokes of wheel - bones of arm" , "They meet in the middle of a circle" .)