Sister of Castor and Pollux (5)
I believe the answer is:
helen
'sister of castor and pollux' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)