Crossword Genius

Confirm aunt is at best confused (12)

Ross

I believe the answer is:

substantiate

'confirm' is the definition.
('substantiate' can be a synonym of 'confirm')

'aunt is at best confused' is the wordplay.
'confused' indicates an anagram.
'aunt'+'is'+'at'+'best'='auntisatbest'
'auntisatbest' with letters rearranged gives 'SUBSTANTIATE'.

(Other definitions for substantiate that I've seen before include "Confirm" , "Prove to be true" , "Prove truth of (charge)" .)

I've seen this clue in The Times.
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