Bird, large one about to be caged? (7)
I believe the answer is:
pelican
'bird' is the definition.
(I know that pelican is a type of pelecaniform seabird)
'large one about to be caged?' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'one' becomes 'i' (Roman numeral).
'about' means one lot of letters goes inside another.
'to be caged?' becomes 'pecan' (I can't justify this - if you can you should believe this answer much more).
'l'+'i'='li'
'li' going into 'pecan' is 'PELICAN'.
(Other definitions for pelican that I've seen before include "Aquatic bird - type of road crossing" , "nickname of US State" , "Seabird having a large bill with a pouch for fish" , "Large-pouched bird" , "Bird's" .)