Radius's neighbor (4)
I believe the answer is:
ulna
'radius's neighbor' 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" , "Inner bone of the human forearm" , "Uneven" , "Car document; ticket book" , "part of anatomy" .)