Bone that parallels the radius (4)
I believe the answer is:
ulna
'bone that parallels 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 "Bone from the human elbow" , "The longer and inner bone in forearm" , "Part of body" , "Thinner of two long bones in forearm" , "One in arms" .)