Collision — shock (6)
I believe the answer is:
impact
I believe this clue is a double definition.
'collision' is the first definition.
(I've seen this in another clue)
'shock' is the second definition.
(I know that shock can be written as impact)
(Other definitions for impact that I've seen before include "The striking of one thing against another" , "Marked effect; hit" , "Force or shock of a collision" , "Forcible contact between two things" , "Strong effect" .)