Bone found in vegan lunch sent back (4)
I believe the answer is:
ulna
'bone' is the definition.
(I know that ulna is a bone)
'in vegan lunch sent back' is the wordplay.
'in' says the answer is hidden in the clue.
'sent back' says the letters should be written backwards.
'anlu' is hidden in the letters of 'vegan lunch'.
'anlu' back-to-front is 'ULNA'.
'found' acts as a link.
(Other definitions for ulna that I've seen before include "One of two bones of the forearm" , "Uneven" , "a section of framework" , "Thinner of two long bones in forearm" , "Long bone in the arm" .)