Hear about a creature (4)
I believe the answer is:
Here is my best explanation:
'a creature' is the definition.
(rhea is a kind of creature)
'hear about' is the subsidiary indication.
'about' indicates anagramming the letters.
'hear' anagrammed gives 'rhea'.
(Other definitions for rhea that I've seen before include "Ostrich-like bird of South America", "Big bird", "Flightless bird - Greek fertility goddess", "A satellite", "Flightless bird -- satellite of Saturn", "Satellite of Saturn", "S. Amer. flightless bird", "Grounded bird", "Fast runner" and "S American bird resembling an ostrich".)