Muse about birds (4)
I believe the answer is:
emus
'birds' is the definition.
(emu is a kind of bird)
'muse about' is the wordplay.
'about' is an anagram indicator.
'muse' with letters rearranged gives 'EMUS'.
(Other definitions for emus that I've seen before include "Aussie birds" , "Fast, but not high-fliers" , "Largest native Australian birds" , "Running birds" , "Muse about flightless Australian birds" .)