Bird originally kept in tiny enclosure (4)
I believe the answer is:
kite
'bird' is the definition.
(kite is a kind of bird)
'originally kept in tiny enclosure' is the wordplay.
'originally' says to take the initial letters.
The initial letters of 'kept in tiny enclosure' is 'KITE'.
(Other definitions for kite that I've seen before include "Flying toy or kind of hawk" , "Toy that flies on end of string" , "Toy; aircraft (dated)" , "Hawk-like bird" , "Plaything flown in the wind" .)