Charlie, over-hasty, causes accident (5)
I believe the answer is:
crash
'accident' is the definition.
(crash is a kind of accident)
'charlie over-hasty' is the wordplay.
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'over-hasty' becomes 'rash' (I've seen this before).
'c'+'rash'='CRASH'
'causes' is the link.
(Other definitions for crash that I've seen before include "Disastrous market fall" , "Collide violently" , "Impact" , "Car accident" , "Dramatic fall" .)