Bone of foul nature (4)
I believe the answer is:
ulna
'bone' is the definition.
(ulna is a kind of bone)
'of foul nature' is the wordplay.
'of' indicates the answer is hidden within the clue.
'ULNA' is hidden within 'foul nature'.
(Other definitions for ulna that I've seen before include "The longer and inner bone in forearm" , "One in arms" , "Part of body" , "Thinner and longer of two bones in forearm" , "part of arm" .)