Prefix with pilot (4)
I believe the answer is:
auto
'prefix with pilot' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for auto that I've seen before include "Motor vehicle, in short" , "Prefix to denote self or same" , "Car (American)" , "Car (mostly US)" , "Prefix denoting by or of the same person or thing" .)