We must admit these birds can't fly (4)
I believe the answer is:
emus
'these birds can't fly' is the definition.
The definition suggests an adverb but the answer is not.
'we must admit' is the wordplay.
'admit' indicates a hidden word (I've seen 'admits' mean this).
'EMUS' is hidden within 'we must'.
Can you help me to learn more?
(Other definitions for emus that I've seen before include "Running birds" , "Muse about flightless Australian birds" , "laboratory units" , "Large flightless birds of Australia" , "Aussie birds" .)