Elbow-wrist connector (4)
I believe the answer is:
'elbow-wrist connector' 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 "Part of body" , "structural member?" , "Uneven" , "Thinner and longer of two bones in forearm" , "Long bone in the arm" .)