Helen's mother
I believe the answer is:
leda
'helen'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 "She was seduced by Zeus disguised as a swan" , "One ravished" , "Zeus turned into a swan to seduce her" , "- - and the Swan (mythology)" , "Mother of Helen and Pollux" .)