Nut keeps a third from little bird with a big bill (7)
I believe the answer is:
pelican
'a big bill' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'nut keeps a third from little bird' is the wordplay.
'nut' becomes 'pecan' (type of nut).
'keeps' means one lot of letters goes inside another.
'a third from little bird' becomes 'li' (I am not sure about this - if you are sure you should believe this answer much more).
'pecan' enclosing 'li' is 'PELICAN'.
'with' acts as a link.
Can you help me to learn more?
(Other definitions for pelican that I've seen before include "Its beak holds more than its belly can" , "Large aquatic bird" , "Aquatic, billed bird" , "Bird, large bag under bill" , "one with huge bill" .)