Hears about birds (5)
I believe the answer is:
rheas
'birds' is the definition.
(I know this)
'hears about' is the wordplay.
'about' indicates anagramming the letters.
'hears' anagrammed gives 'RHEAS'.
(Other definitions for rheas that I've seen before include "these birds won't fly" , "Flightless birds" , "Ostrich-like birds" .)