Mother of Castor and Pollux (4)
I believe the answer is:
leda
'mother of castor and pollux' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for leda that I've seen before include "- - and the Swan (mythology)" , "Queen of Sparta, mother of Helen of Troy" , "Zeus turned into a swan to seduce her" , "She was seduced by Zeus disguised as a swan" , "Mother of Helen and Pollux" .)