In addition, bird has a breastbone (7)
I believe the answer is:
sternum
'a breastbone' is the definition.
(synonyms)
'in addition bird' is the wordplay.
'in' means one lot of letters goes inside another.
'addition' becomes 'sum' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'bird' becomes 'tern' (tern is a kind of bird).
'sum' placed around 'tern' is 'STERNUM'.
'has' acts as a link.
Can you help me to learn more?
(Other definitions for sternum that I've seen before include "front of breast" , "Munster (anag.)" , "Chest bone" , "big bone" , "Breastbone" .)