Aviary (4)
I believe the answer is:
cage
'aviary' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cage that I've seen before include "Enclosure made from metal bars" , "Box of bars to confine an animal" , "Enclose with bars" , "Barred accommodation for animal" , "In places squirrel's nest" .)