Collide (5)
I believe the answer is:
crash
'collide' is the definition.
(I know that crash is a more specific form of the action collide)
This is the entire clue.
(Other definitions for crash that I've seen before include "Planned quickly for emergency" , "Economic collapse" , "(Of shares) fall suddenly by a great amount" , "Disastrous market fall" , "Bang" .)