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