Bone rigid? I'm not so sure (7)
I believe the answer is:
sternum
'bone' is the definition.
(sternum is a kind of bone)
'rigid? i'm not so sure' is the wordplay.
'rigid?' becomes 'stern' (I've seen this before).
'i'm not so sure' becomes 'um'.
'stern'+'um'='STERNUM'
(Other definitions for sternum that I've seen before include "Breastbone" , "somewhere in chest" , "big bone" , "Chest bone" , "Human bone -- ten rums (anag)" .)