Twins for instance backed the car (6)
I believe the answer is:
gemini
'twins' is the definition.
(twins are the symbol of the star sign Gemini)
'for instance backed the car' is the wordplay.
'for instance' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'backed' says the letters should be written backwards.
'the car' becomes 'mini' (Mini Cooper).
'eg' reversed gives 'ge'.
'ge'+'mini'='GEMINI'
(Other definitions for gemini that I've seen before include "Sign of the Twins ( 6)" , "space programme" , "star group" , "Third sign of the Zodiac" , "Zodiacal twins" .)