Parent of Poseidon (4)
I believe the answer is:
rhea
'parent of poseidon' is the definition.
(I know that Rhea child Poseidon)
This is the entire clue.
(Other definitions for rhea that I've seen before include "Fertility goddess in ancient Greek mythology" , "Greek goddess of fertility - running bird" , "Titaness" , "New World flightless bird" , "Grounded bird" .)