Zeus' mother (4)
I believe the answer is:
rhea
'zeus' mother' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rhea that I've seen before include "Ostrich-like bird of South America" , "New World ostrich" , "South American bird resembling the ostrich" , "Fast runner" , "Fast-running flightless bird" .)