In addition, bird has breastbone (7)
I believe the answer is:
sternum
'breastbone' is the definition.
(synonyms)
'in addition bird' is the wordplay.
'in' indicates putting letters inside.
'addition' becomes 'sum' (I am not sure about this - if you are sure you should believe this answer much more).
'bird' becomes 'tern' (tern is a kind of bird).
'sum' enclosing 'tern' is 'STERNUM'.
'has' is the link.
Can you help me to learn more?
(Other definitions for sternum that I've seen before include "big bone" , "Human bone -- ten rums (anag)" , "front of breast" , "somewhere in chest" , "Chest bone" .)