Driver's aid (3,3)
'driver's aid' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for satnav that I've seen before include "Driver's route finder" , "course-finding apparatus" , "Electronic direction finder" , "Route guide" , "Driver's guide" .)
(Other definitions for sat nav that I've seen before include "Guidance system" , "Direction-finding equipment" , "Aid" .)