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' is an insertion indicator (inserted letters break into the word).
'a little' becomes 'abit' (synonyms).
'back' is a reversal indicator.
'i' inserted within 'abit' is 'aibit'.
'aibit' backwards is 'TIBIA'.
(Other definitions for tibia that I've seen before include "Bone of the lower leg" , "Bone in osso bucco" , "The formal shinbone" , "The large shinbone" , "part of skeleton" .)