The twin of Castor (6)
I believe the answer is:
pollux
'the twin of castor' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pollux that I've seen before include "It's close to Castor" , "One half of Gemini" , "Heavenly twin" , "Brightest star in Gemini" , "Bright star in constellation of Gemini, brother to Castor" .)