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