Bone is hard? I'm pondering (7)
I believe the answer is:
sternum
'bone' is the definition.
(sternum is a kind of bone)
'hard? i'm pondering' is the wordplay.
'hard?' becomes 'stern' (**).
'i'm pondering' becomes 'um' (expression of hesitation).
'stern'+'um'='STERNUM'
'is' acts as a link.
(Other definitions for sternum that I've seen before include "big bone" , "Human bone -- ten rums (anag)" , "front of breast" , "Chest bone" , "somewhere in chest" .)