Important document provides after-dinner instruction? (8)

I believe the answer is:

passport

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'important document' is the definition.
'passport' can be an answer for 'document' (passport is a kind of document). I'm unsure of the 'important' bit.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for passport that I've seen before include "Document to allow entry to another country" , "Authorization to go abroad and return" , "entry permit?" , "Travel identification" , "Traveller's aid" .)

I've seen this clue in The Independent.

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