Collision and effect (6)
I believe the answer is:
impact
This clue is a double definition.
'collision' is the first definition.
(I've seen this before)
'effect' is the second definition.
(I know that impact is a type of effect)
'and' acts as a link.
(Other definitions for impact that I've seen before include "Force of blow or collision" , "Marked effect; hit" , "hit, in a big way" , "Impression" , "Forceful striking" .)