Cain's mother (3)
I believe the answer is:
eve
'cain's mother' is the definition.
(Cain and Abel were sons of Adam and Eve)
This is the entire clue.
(Other definitions for eve that I've seen before include "Day before; first woman" , "comes before Jan. 1st" , "Adam's partner" , "when it gets dark" , "She reportedly lived first" .)