Risk a car crash, when two metals collide? (3,2)
I believe the answer is:
cut in
'risk a car crash' is the definition.
(as in cutting in when overtaking a vehicle)
I can't explain the remainder of the clue.
(Other definitions for cut in that I've seen before include "Interpose" , "Interrupt a conversation" , "Interrupt someone speaking" , "interrupt speaker" , "Interrupt; veer across another vehicle and take its space" .)