Arm bones (5)
I believe the answer is:
radii
'arm bones' is the definition.
(radius is a kind of arm bone)
This is the entire clue.
(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" .)