Birds are musical? Not all (4)
I believe the answer is:
emus
'birds' is the definition.
(emu is a kind of bird)
'are musical? not all' is the wordplay.
'not all' says the answer is hidden in the clue (only some of the letters).
'EMUS' is hidden in the letters of 'are musical'.
(Other definitions for emus that I've seen before include "Aussie birds" , "Muse about flightless Australian birds" , "Fast, but not high-fliers" , "Large flightless birds of Australia" , "Running birds" .)