Bone parallel to the radius (4)
I believe the answer is:
ulna
'bone parallel to the radius' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ulna that I've seen before include "Radius partner" , "Part of bone-structure" , "part of arm" , "One of two bones of the forearm" , "Part of body" .)