Bone from three in the reckoning? (7)
I believe the answer is:
sternum
'bone' is the definition.
(the breastbone)
'three in the reckoning?' is the wordplay.
I cannot really see how this works, but
'three' could be 'ter' (see Chambers) and 'ter' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'from' is the link.
Can you help me to learn more?
(Other definitions for sternum that I've seen before include "front of breast" , "Human bone -- ten rums (anag)" , "Chest bone" , "Munster (anag.)" , "somewhere in chest" .)