Agree and a small double, neat, is poured out (6)

I believe the answer is:

assent

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'agree' is the definition.
(I know that assent is a type of agree)

I don't understand the rest of the clue.

(Other definitions for assent that I've seen before include "Agree - concede" , "Official sanction" , "Give the thumbs-up" , "Say yes" , "Agreement (to a proposal)" .)

I've seen this clue in the Evening Standard.

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