Rachel's sister
I believe the answer is:
'rachel's sister' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for leah that I've seen before include "Jacob's first wife (Bible)" , "First wife of Jacob" , "Jacobs first wife (Gen.29)" , "Rachel's sister" , "Hale (anag) - wife of Jacob" .)