Some plane! Must be a flightless one (3)
I believe the answer is:
emu
'a flightless one' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'some plane must be' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'EMU' is hidden in the letters of 'plane must be'.
Can you help me to learn more?
(Other definitions for emu that I've seen before include "Long-necked bird" , "Physics unit" , "Big bird" , "it'll never fly!" , "Ostrich-like bird" .)