Regularly pulling at bone (4)
I believe the answer is:
ulna
'bone' is the definition.
(I know that ulna is a bone)
'regularly pulling at' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'pulling'+'at'='pullingat'
The alternate letters of 'pullingat' are 'ULNA'.
(Other definitions for ulna that I've seen before include "Part of body" , "Long bone in the arm" , "part of arm" , "Part of bone-structure" , "One in arms" .)