Guidance system positioned vehicle in the wrong direction (6)
I believe the answer is:
satnav
'guidance system' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'positioned vehicle in the wrong direction' is the wordplay.
'positioned' becomes 'sat' (sitting is a kind of positioning).
'vehicle' becomes 'van' (type of vehicle).
'in the wrong direction' is a reversal indicator.
'van' back-to-front is 'nav'.
'sat'+'nav'='SATNAV'
Can you help me to learn more?
(Other definitions for satnav that I've seen before include "course-finding apparatus" , "Navigation system (abbrev.)" , "one may help the driver" , "Device for finding one's way" , "Road atlas replacer" .)