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