Bone in grave Greek character returned (7)
I believe the answer is:
sternum
'bone' is the definition.
(the breastbone)
'grave greek character returned' is the wordplay.
'grave' becomes 'stern' (I've seen this in another clue).
'greek character' becomes 'mu' (letter from the Greek alphabet).
'returned' shows that the letters should be reversed in order.
'mu' back-to-front is 'um'.
'stern'+'um'='STERNUM'
'in' is the link.
(Other definitions for sternum that I've seen before include "Breastbone" , "Chest bone" , "somewhere in chest" , "Human bone -- ten rums (anag)" , "Munster (anag.)" .)