Sign, for example, mounted on car (6)
I believe the answer is:
gemini
'sign' is the definition.
(Gemini is a star sign)
'example mounted on car' is the wordplay.
'example' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'mounted' says the letters should be written in reverse (in a down clue, letters mounted up).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'car' becomes 'mini' (Mini Cooper).
'eg' back-to-front is 'ge'.
'ge'+'mini'='GEMINI'
'for' acts as a link.
(Other definitions for gemini that I've seen before include "set of twinklers" , "House" , "Twins star sign" , "May person" , "Twins star-sign" .)