Twins, say, getting up on car (6)
I believe the answer is:
gemini
'twins' is the definition.
(twins are the symbol of the star sign Gemini)
'say getting up on car' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'getting up' says the letters should be written backwards.
'on' says to put letters next to each other.
'car' becomes 'mini' (Mini Cooper).
'eg' backwards is 'ge'.
'ge'+'mini'='GEMINI'
(Other definitions for gemini that I've seen before include "space programme" , "Third zodiacal sign" , "lot of stars" , "Castor and Pollux constellation" , "Zodiacal twins" .)