How the warder said "Files are not allowed in cells"? (9)

I believe the answer is:

raspingly

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'how' is the definition.
Both the answer and definition are adverbs. Maybe there's a link between them I don't understand?

'warder said files are not allowed in cells?' is the wordplay.
I cannot really see how this works, but
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is present in the answer.
'in' is within the answer.
This may be the basis of the clue (or it may be nonsense).

'the' is the link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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