Genet's a writer let out of prison? (5)

I believe the answer is:

unpen

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'genet's a writer let' is the definition.
I can't judge whether this defines the answer.

'out of prison?' is the wordplay.
'out of' becomes 'un' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'prison?' becomes 'pen' (I've seen this in another clue).
'un'+'pen'='UNPEN'

Can you help me to learn more?

I've seen this clue in The Telegraph.

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