Does it stimulate you to be a bit nice? (5)

I believe the answer is:

tonic

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'nice?' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?

I can't explain the remainder of the clue.

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(Other definitions for tonic that I've seen before include "A pick-me-up drink, nice with gin" , "__water; gin mixer" , "Quinine drink; keynote" , "An enlivener" , "medicinal preparation" .)

I've seen this clue in the Evening Standard.

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