Longest armbone (4)
I believe the answer is:
ulna
'longest armbone' 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 arm" , "Inner bone of the human forearm" , "Thinner of two long bones in forearm" , "The longer and inner bone in forearm" , "Part of bone-structure" .)