Mother of Demeter (4)
I believe the answer is:
rhea
'mother of demeter' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rhea that I've seen before include "Fertility goddess in ancient Greek mythology" , "China grass" , "New World flightless bird" , "Flightless bird from South America" , "nonflyer" .)