Bird in addition reveals chest part (7)
I believe the answer is:
sternum
'chest part' is the definition.
('sternum' is part of 'chest')
'bird in addition' is the wordplay.
'bird' becomes 'ern' (ern is a kind of bird).
'in' means one lot of letters goes inside another.
'addition' becomes 'stum' (I can't explain this - if you can you should give a lot more credence to this answer).
'ern' inserted inside 'stum' is 'STERNUM'.
'reveals' is the link.
Can you help me to learn more?
(Other definitions for sternum that I've seen before include "Chest bone" , "Munster (anag.)" , "Breastbone" , "front of breast" , "somewhere in chest" .)