One breaks a little back-bone (5)
I believe the answer is:
tibia
'bone' is the definition.
(tibia is a kind of bone)
'one breaks a little back' is the wordplay.
'one' becomes 'i' (Roman numeral).
'breaks' means one lot of letters goes inside another.
'a little' becomes 'abit' ('a bit' can be a synonym of 'a little'**).
'back' says the letters should be written backwards.
'i' put into 'abit' is 'aibit'.
'aibit' in reverse letter order is 'TIBIA'.
(Other definitions for tibia that I've seen before include "Bone in osso bucco" , "Shin bone" , "Bone of the lower leg" , "Part of leg of" , "The large shinbone" .)