If edgy trio gets panicky in the middle, this will do them good (5)

I believe the answer is:

tonic

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'good' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

(Other definitions for tonic that I've seen before include "Producing tension of muscles" , "Keynote of a scale" , "Accompaniment to gin?" , "Stimulant" , "__water; gin mixer" .)

I've seen this clue in The Irish Times.

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