Bone not yielding useful material initially (7)
I believe the answer is:
sternum
'bone' is the definition.
(sternum is a kind of bone)
'not yielding useful material initially' is the wordplay.
'not yielding' becomes 'stern'.
'initially' indicates taking the first letters.
The initial letters of 'useful material' is 'um'.
'stern'+'um'='STERNUM'
(Other definitions for sternum that I've seen before include "big bone" , "Breastbone" , "somewhere in chest" , "Munster (anag.)" , "front of breast" .)