Twins say overturned car (6)
I believe the answer is:
gemini
'twins' is the definition.
(twins are the symbol of the star sign Gemini)
'say overturned car' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'overturned' is a reversal indicator.
'car' becomes 'mini' (Mini Cooper).
'eg' back-to-front is 'ge'.
'ge'+'mini'='GEMINI'
(Other definitions for gemini that I've seen before include "Twins star sign" , "Sign of the Twins ( 6)" , "Twins constellation" , "Zodiacal twins" , "Heavenly sign of the Twins" .)