A sign of twins (6)
I believe the answer is:
gemini
I believe this clue is a double definition.
'a sign' is the first definition.
(Gemini is a star sign)
'twins' is the second definition.
(twins are the symbol of the star sign Gemini)
'of' is the link.
(Other definitions for gemini that I've seen before include "Twins constellation" , "space programme" , "House" , "Third zodiacal sign" , "Castor and Pollux constellation" .)