Wonderful Natalie - not all bone! (4)
I believe the answer is:
'bone' is the definition.
(ulna is a kind of bone)
'wonderful natalie not all' is the wordplay.
'not all' indicates a hidden word.
'ULNA' is hidden in the letters of 'wonderful natalie'.
(Other definitions for ulna that I've seen before include "Part of bone-structure" , "Radius partner" , "The longer and inner bone in forearm" , "part of arm" , "structural member?" .)