Rear parking collision (4)
I believe the answer is:
bump
'collision' is the definition.
(I've seen this before)
'rear parking' is the wordplay.
'rear' becomes 'bum' (synonyms).
'parking' becomes 'p' (abbreviation on road signs).
'bum'+'p'='BUMP'
(Other definitions for bump that I've seen before include "Jolting movement or skin swelling" , "Prang" , "Impact injury" , "Thud down, into" , "Slang for kill - . . . . off" .)