A bird cage (3)

I believe the answer is:

pen

I believe this is a double definition.

'a bird' is the first definition.
(pen is a kind of bird)

'cage' is the second definition.
(both can mean an enclosure)

(Other definitions for pen that I've seen before include "Writing tool" , "International writers's organisation" , "Prison" , "Cage - bird" , "Confine the writer" .)

I've seen this clue in The Globe and Mail.

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