Medicine lacking what it takes to be too nice (5)

I believe the answer is:

tonic

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'medicine' is the definition.
(tonic is a kind of medicine)

I don't understand how the rest of the clue works.

(Other definitions for tonic that I've seen before include "Invigorating drink" , "makes you feel good?" , "Keynote of a scale" , "Gin mixer - pick-me-up" , "something medicinal" .)

I've seen this clue in the Evening Standard.

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