Hears about birds (5)
I believe the answer is:
rheas
'birds' is the definition.
(rhea is a kind of bird)
'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" .)