Big Bird is a dish! (4)
I believe the answer is:
bowl
'dish' is the definition.
(I know that Bowl is a bowl)
'big bird' is the wordplay.
'big' becomes 'b' (I can't justify this - if you can you should give a lot more credence to this answer).
'bird' becomes 'owl' (owl is a kind of bird).
'b'+'owl'='BOWL'
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for bowl that I've seen before include "Greatly surprise" , "Soup dish - roll a ball" , "food container" , "Basin; attack wicket" , "US stadium for sporting or musical events" .)