Help for driver reversing vehicle after test (6)
I believe the answer is:
satnav
'help for driver' is the definition.
(satellite navigation device)
'reversing vehicle after test' is the wordplay.
'reversing' is a reversal indicator.
'vehicle' becomes 'van' (van is a kind of vehicle).
'after' means one lot of letters go next to another.
'test' becomes 'SAT' (SAT is a type of exam).
'van' in reverse letter order is 'nav'.
'nav' after 'sat' is 'SATNAV'.
(Other definitions for satnav that I've seen before include "Routemaster perhaps" , "Guide for driver" , "Savant (anag)" , "'Guidance system, in short (3-3)'" , "Road atlas replacer" .)