Bone problem restricts bird (7)
I believe the answer is:
sternum
'bone' is the definition.
(the breastbone)
'problem restricts bird' is the wordplay.
'problem' becomes 'sum' (arithmetic problem).
'restricts' means one lot of letters goes inside another.
'bird' becomes 'tern' (tern is a kind of bird).
'sum' placed around 'tern' is 'STERNUM'.
(Other definitions for sternum that I've seen before include "Breastbone" , "Chest bone" , "Human bone -- ten rums (anag)" , "front of breast" , "big bone" .)