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