Risk a car crash, when two metals collide? (3,2)
'risk a car crash' is the definition.
(as in cutting in when overtaking a vehicle)
I don't understand the rest 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".)