Crossword Genius

What must be left at the end of the sentence (6)

Ross

I believe the answer is:

prison

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

'end of the sentence' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I don't see?

'what must be left at the' is the wordplay.
I cannot really see how this works, but
'at' could be 'on' (location) and 'on' is found 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 "porridge" , "Clink - jug" , "Cooler" , "Jail" , "Gaol" .)

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