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 "Large S. American bird", "Fast-running flightless bird", "South American bird resembling the ostrich", "Ostrich-like bird of South America", "S American bird resembling an ostrich", "New World flightless bird", "S. Amer. flightless bird", "Mother of Zeus; a flightless bird", "Flightless bird from South America" and "Large flightless bird".)