Bird’s extremely efficient food passage (6)
I believe the answer is:
gullet
'food passage' is the definition.
'gullet' can be an answer for 'passage' (gullet is a kind of passage). I am not sure about the 'food' bit.
'bird's extremely efficient' is the wordplay.
'bird' becomes 'gull' (gull is a kind of bird).
'extremely' says to hollow out the word (remove centre letters).
'efficient' with its centre removed is 'et'.
'gull'+'et'='GULLET'
Can you help me to learn more?
(Other definitions for gullet that I've seen before include "Food-passage from mouth to stomach" , "part of canal" , "Food passage in the throat" , "Oesophagus" , "Food passage from mouth to stomach" .)