A little backward eating one's bone (5)
'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 "ancient pipe", "The large shinbone", "Bone of the lower leg", "Bone in osso bucco", "Part of leg of".)