Gemini twin (6)
I believe the answer is:
castor
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">gemini twin</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)