Stimulant, often after a gin (5)

I believe the answer is:

tonic

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'stimulant' is the definition.
(I've seen this in another clue)

I cannot understand how the rest of the clue works.

(Other definitions for tonic that I've seen before include "something invigorating" , "Accompaniment to gin?" , "Important note" , "Invigorating drink" , "requirement for invigoration?" .)

I've seen this clue in The Mirror.

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