An Italian could start something stimulating (5)

I believe the answer is:

tonic

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'stimulating' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for tonic that I've seen before include "Invigorating medicine - keynote" , "Producing tension of muscles" , "Carbonated mixer" , "Keynote of a scale" , "requirement for invigoration?" .)

I've seen this clue in the Evening Standard.

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