Driving aid positioned in front of reversing vehicle (6)
I believe the answer is:
satnav
'driving aid' is the definition.
(I've seen this before)
'positioned in front of reversing vehicle' is the wordplay.
'positioned' becomes 'sat' (sitting is a kind of positioning).
'in front of' is a charade indicator (letters next to each other).
'reversing' is a reversal indicator.
'vehicle' becomes 'van' (van is a kind of vehicle).
'van' reversed gives 'nav'.
'sat'+'nav'='SATNAV'
(Other definitions for satnav that I've seen before include "Navigation system (abbrev.)" , "one may help the driver" , "Driver's aid" , "Electronic direction finder" , "Routemaster perhaps" .)