Bone in back? That's doubtful (7)
I believe the answer is:
sternum
'bone' is the definition.
(the breastbone)
'back? that's doubtful' is the wordplay.
'back?' becomes 'stern' (stern means the back of a boat).
'that's doubtful' becomes 'um' (saying 'um' communicates doubt).
'stern'+'um'='STERNUM'
'in' is the link.
(Other definitions for sternum that I've seen before include "Munster (anag.)" , "Chest bone" , "big bone" , "front of breast" , "somewhere in chest" .)