It's the devil to turn when there's a collision (6)
I believe the answer is:
impact
'there's a collision' is the definition.
(I've seen this in another clue)
'it's the devil to turn' is the wordplay.
'it's the devil' becomes 'imp' (devilish creature).
'to turn' becomes 'act' (theatre).
'imp'+'act'='IMPACT'
'when' acts as a link.
(Other definitions for impact that I've seen before include "Marked effect; hit" , "Forceful contact between two objects" , "Result" , "Bear upon" , "The striking of one thing against another" .)