Woman securing Labrador’s first bone (4)
I believe the answer is:
ulna
'bone' is the definition.
(I know that ulna is a bone)
'woman securing labrador's first' is the wordplay.
'woman' becomes 'una' ().
'securing' means one lot of letters goes inside another.
'labrador's first' becomes 'l' (ust letter of 'labrador').
'una' enclosing 'l' is 'ULNA'.
(Other definitions for ulna that I've seen before include "One of two bones of the forearm" , "a section of framework" , "structural member?" , "part of anatomy" , "Long bone in the arm" .)