Bone next to the radius (4)
I believe the answer is:
ulna
'bone next to the radius' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ulna that I've seen before include "part of anatomy" , "Long bone in the arm" , "Inner bone of the human forearm" , "Uneven" , "Thinner of two long bones in forearm" .)