Sign, say, over vehicle (6)
I believe the answer is:
gemini
'sign' is the definition.
(Gemini is an example)
'say over vehicle' is the wordplay.
'say' becomes 'eg'.
'over' is a reversal indicator.
'vehicle' becomes 'mini' (I've seen this before**).
'eg' back-to-front is 'ge'.
'ge'+'mini'='GEMINI'
(Other definitions for gemini that I've seen before include "Zodiac sign - the Twins" , "Twins star-sign" , "Twins constellation" , "Third zodiacal sign" , "Sign of the Twins ( 6)" .)