Mother of Helen of Troy
I believe the answer is:
'mother of helen of troy' 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" , "She was seduced by Zeus disguised as a swan" , "- - and the Swan (mythology)" , "Zeus turned into a swan to seduce her" , "Queen of Sparta, mother of Helen of Troy" .)