Bone in back? Er... ! (7)
I believe the answer is:
sternum
'bone' is the definition.
(sternum is a kind of bone)
'back? er' is the wordplay.
'back?' becomes 'stern' (stern is a kind of back).
'er' becomes 'um'.
'stern'+'um'='STERNUM'
'in' is the link.
(Other definitions for sternum that I've seen before include "Munster (anag.)" , "Breastbone" , "Chest bone" , "big bone" , "front of breast" .)