Crossword Genius

Cage bird (6)

Ross

I believe the answer is:

prison

I'm a little stuck... Click here to teach me more about this clue!

This might be a double definition.

'cage' is the first definition.
(I've seen this in another clue)

'bird' is the second definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?

Can you help me to learn more?

(Other definitions for prison that I've seen before include "Place of incarceration" , "Place of confinement" , "Correctional institution" , "Stir - can" , "Place of detention for criminals" .)

I've seen this clue in The Telegraph and The Sun.
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