American author takes in quiet marginal note (9)

I believe the answer is:

apostille

'marginal note' is the definition.
(type of authentication mark)

'american author takes in quiet' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'author' becomes 'poe' (Edgar Allan Poe).
'takes in' means one lot of letters goes inside another.
'quiet' becomes 'still' (still is a kind of quiet).
'poe' placed around 'still' is 'postille'.
'a'+'postille'='APOSTILLE'

I've seen this clue in The Guardian.

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