An engine, or the driver (5)
I believe the answer is:
motor
This clue is a double definition.
'an engine' is the first definition.
(I've seen this before)
'the driver' is the second definition.
(I've seen this before)
'or' is the link.
(Other definitions for motor that I've seen before include "(Transport using) engine" , "Make good progress" , "Engine of vehicle" , "Move fast" , "Car; travel quickly" .)