Bone that is broken in accident, first of all (5)
I believe the answer is:
tibia
'bone' is the definition.
(tibia is a kind of bone)
'that is broken in accident first of all' is the wordplay.
'first of all' says to take the initial letters.
The initial letters of 'that is broken in accident' is 'TIBIA'.
(Other definitions for tibia that I've seen before include "part of skeleton" , "ancient pipe" , "Bone of the lower leg" , "Part of leg of" , "The formal shinbone" .)