Bone, slightly backwards, covering one (5)
I believe the answer is:
tibia
'bone' is the definition.
(I know that tibia is a bone)
'slightly backwards covering one' is the wordplay.
'slightly' becomes 'abit' (I've seen this before).
'backwards' shows that the letters should be reversed in order.
'covering' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'abit' back-to-front is 'tiba'.
'tiba' placed around 'i' is 'TIBIA'.
(Other definitions for tibia that I've seen before include "The formal shinbone" , "part of skeleton" , "Bone in osso bucco" , "The large shinbone" , "Bone of the lower leg" .)