Careful, naturally, to bury this bone (4)
I believe the answer is:
ulna
'this bone' is the definition.
(ulna is a kind of bone)
'careful naturally to bury' is the wordplay.
'to bury' indicates a hidden word.
'ULNA' is hidden in the letters of 'careful naturally'.
(Other definitions for ulna that I've seen before include "Bone from the human elbow" , "Part of body" , "Thinner and longer of two bones in forearm" , "The longer and inner bone in forearm" , "structural member?" .)