Umpire rested half her group and prescribed a tonic (9)

I believe the answer is:

refresher

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'tonic' is the definition.
(thesaurus)

'umpire rested half her' is the wordplay.
'umpire' becomes 'ref' (short for referee).
'half' indicates taking half.
'rested' cut in half is 'res'.
'ref'+'res'+'her'='REFRESHER'

'group and prescribed a' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.

Can you help me to learn more?

(Other definitions for refresher that I've seen before include "Sort of course" , "Follow-up course; extra fee in long court case" , "drink" , "This reinvigorates" , "course of sorts" .)

I've seen this clue in the Sydney Morning Herald.

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