Stimulant, often after gin (5)

I believe the answer is:

tonic

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'stimulant' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for tonic that I've seen before include "something medicinal" , "__water; gin mixer" , "Invigorating drink" , "something nice and fresh" , "Quinine water" .)

I've seen this clue in The Mirror.

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