Radius partner (4)
I believe the answer is:
ulna
'radius partner' is the definition.
(I've seen this before)
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" , "Part of bone-structure" , "a section of framework" , "One in arms" .)