Collision causes little devil to do something (6)
I believe the answer is:
impact
'collision' is the definition.
(I've seen this in another clue)
'little devil to do something' is the wordplay.
'little devil' becomes 'imp' (an imp is a little demon).
'to' says to put letters next to each other.
'do something' becomes 'act' (to act is to do something).
'imp'+'act'='IMPACT'
'causes' is the link.
(Other definitions for impact that I've seen before include "The striking of one body against another" , "Collision - effect" , "Forceful striking" , "The striking of one thing against another" , "Forcible contact; marked influence" .)