Bone of the forearm (4)
I believe the answer is:
ulna
'bone of the forearm' 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 "The longer and inner bone in forearm" , "Thinner of two long bones in forearm" , "Inner bone of the human forearm" , "One of two bones of the forearm" , "Long bone in the arm" .)