Hears about birds unable to fly (5)
I believe the answer is:
Here is my best explanation:
'birds unable to fly' is the definition.
I can't judge whether this definition defines the answer.
'hears about' is the wordplay.
'about' indicates anagramming the letters.
'hears' anagrammed gives 'RHEAS'.
(Other definitions for rheas that I've seen before include "Ostrich-like birds", "these birds won't fly", "Flightless birds" and "Birds".)