Crossword Genius

Where you'll find screws on the doors (6)

Ross

I believe the answer is:

prison

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

'where you'll find screws' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'on the doors' is the wordplay.
I cannot quite understand how this works, but
'on' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for prison that I've seen before include "Clink - jug" , "cage" , "Nick" , "No rips (anag)" , "punishment" .)

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