A little backward eating one's bone (5)
I believe the answer is:
tibia
'bone' is the definition.
(tibia is a kind of bone)
'a little backward eating one's' is the wordplay.
'a little' becomes 'abit' ('a bit' can be a synonym of 'a little'**).
'backward' shows that the letters should be reversed in order.
'eating' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'abit' written backwards gives 'tiba'.
'tiba' enclosing 'i' is 'TIBIA'.
(Other definitions for tibia that I've seen before include "The large shinbone" , "ancient pipe" , "Shin bone" , "The formal shinbone" , "Bone of the lower leg" .)