Lunar fracture of a bone (5)
I believe the answer is:
ulnar
'of a bone' is the definition.
(relating to the ulna, an arm bone)
'lunar fracture' is the wordplay.
'fracture' indicates anagramming the letters (I've seen 'fractured' mean this).
'lunar' anagrammed gives 'ULNAR'.
(Other definitions for ulnar that I've seen before include "Relating to one of the bones of the forearm" , "of certain bones" , "Relating to a particular arm bone" , "Relating to a bone in the forearm" .)