Confirm aunt is at best confused (12)
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)" .)