Helen's mother (4)
I believe the answer is:
'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" .)