Sister of Castor and Pollux (5)
I believe the answer is:
helen
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sister of castor and pollux</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for helen that I've seen before include "-- Mirren, English actress" , "Greek woman famed for beauty" , "Famous beauty of Troy" , "Spartan beauty" , "Woman taken to Troy by Paris" .)