Add "Inside there's a bird bone" (7)
I believe the answer is:
sternum
'bone' is the definition.
(sternum is a kind of bone)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sternum that I've seen before include "Human bone -- ten rums (anag)" , "front of breast" , "Breastbone" , "somewhere in chest" , "big bone" .)