Bird, one sadly missing out — it can't fly! (3)
I believe the answer is:
pig
'it can't fly' is the definition.
(pigs famously can't fly)
I can't explain the remainder of the clue.
(Other definitions for pig that I've seen before include "Difficult task" , "Eg, sow" , "Farmyard animal" , "Boar or sow" , "Mass of metal simply cast" .)