Mistakenly hear: “Bird!” (4)
I believe the answer is:
rhea
'bird' is the definition.
(rhea is a kind of bird)
'mistakenly hear' is the wordplay.
'mistakenly' indicates anagramming the letters.
'hear' is an anagram of 'RHEA'.
(Other definitions for rhea that I've seen before include "Large S. American bird" , "Titaness" , "Flightless bird from South America" , "South American flightless bird - mother of Zeus" , "Grounded bird" .)