Accidentally, I bit a bone (5)
I believe the answer is:
tibia
'bone' is the definition.
(I know that tibia is a bone)
'accidentally i bit a' is the wordplay.
'accidentally' indicates anagramming the letters (a chaotic or accidental ordering of the letters).
'i'+'bit'='ibit'
'ibit' with letters rearranged gives 'tibi'.
'tibi'+'a'='TIBIA'
(Other definitions for tibia that I've seen before include "Bone in osso bucco" , "Part of leg of" , "The large shinbone" , "Bone of the lower leg" , "Shin bone" .)