Mother of Clytemnestra (4)
I believe the answer is:
leda
'mother of clytemnestra' 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 "small satellite of Jupiter" , "Zeus turned into a swan to seduce her" , "She was seduced by Zeus disguised as a swan" , "- - and the Swan (mythology)" , "Mother of Helen and Pollux" .)