Crossword Genius

Initially, 20% of estuary disappeared in the drink (8)

Ross

I believe the answer is:

vermouth

'the drink' is the definition.
(vermouth is a kind of drink)

I cannot understand how the rest of the clue works.

(Other definitions for vermouth that I've seen before include "Drink" , "Flavoured wine used in cocktails" , "It, possibly" , "Flavoured wine often mixed with gin" , "Herb-flavoured wine drunk with gin" .)

I've seen this clue in The Independent.
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