He must be hiding the bird (3)
I believe the answer is:
emu
'the bird' is the definition.
(emu is a kind of bird)
'he must be hiding' is the wordplay.
'be hiding' says the answer is hidden in the clue.
'EMU' is hidden in the letters of 'he must'.
(Other definitions for emu that I've seen before include "Fast runner in Oz" , "Aussie runner" , "Physics unit" , "Flightless Aus. bird" , "Hull's character" .)