Unlucky accident (6)
I believe the answer is:
mishap
'unlucky accident' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mishap that I've seen before include "Accident, unlucky happening" , "Unpleasant accident" , "Accident, something going wrong" , "An accident or unlucky event" , "'Bad luck, mischance (6)'" .)