Break by accident (5)
I believe the answer is:
smash
I believe this clue is a double definition.
'break' is the first definition.
(smashing is a kind of breaking)
'accident' is the second definition.
(smash is a kind of accident)
'by' is the link.
(Other definitions for smash that I've seen before include "something very popular" , "tremendous success!" , "Wreck" , "Overarm volley" , "Shatter, break up" .)