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 tell whether this defines the answer.
'hears about' is the subsidiary indication.
'about' is an anagram indicator.
'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".)