Crossword Genius

Sent to prison but not at once (5,4)

Ross

I believe the answer is:

given time

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'not at once' is the definition.
I know nothing about this answer so I can't tell whether this works.

'sent to prison' is the wordplay.
'sent to' becomes 'given' (I can't explain this - if you can you should believe this answer much more).
'prison' becomes 'time' (as in a spell in prison).
'given'+'time'='GIVEN TIME'

'but' is the link.

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