Gemini twin (6)
I believe the answer is:
castor
'gemini twin' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for castor that I've seen before include "Roller on chair" , "Small swivelling wheel on a furniture leg" , "Powdered sugar" , "Small swivelled wheel fixed to each leg of a piece of furniture" , "Brother to Pollux" .)