Bone in bird swallowed by tot (7)
I believe the answer is:
sternum
'bone' is the definition.
(the breastbone)
'bird swallowed by tot' is the wordplay.
'bird' becomes 'tern' (tern is a kind of bird).
'swallowed by' is an insertion indicator.
'tot' becomes 'sum' (synonyms).
'tern' placed within 'sum' is 'STERNUM'.
'in' is the link.
(Other definitions for sternum that I've seen before include "Munster (anag.)" , "big bone" , "Breastbone" , "somewhere in chest" , "Chest bone" .)