Twins, say, going north by car (6)
I believe the answer is:
gemini
'twins' is the definition.
(twins are the symbol of the star sign Gemini)
'say going north by car' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'going north' says the letters should be written backwards.
'by' means one lot of letters go next to another.
'car' becomes 'mini' (Mini Cooper).
'eg' reversed gives 'ge'.
'ge'+'mini'='GEMINI'
(Other definitions for gemini that I've seen before include "May person" , "Third sign of the Zodiac" , "Sign of the Twins ( 6)" , "Twins star sign" , "Zodiac sign - the Twins" .)