Confirm aunt is at best confused (12)
I believe the answer is:
Here is my best explanation:
'confirm' is the definition.
('substantiate' can be a synonym of 'confirm')
'aunt is at best confused' is the wordplay.
'confused' indicates an anagram.
'auntisatbest' is an anagram of 'substantiate'.
(Other definitions for substantiate that I've seen before include "Confirm", "Prove to be true" and "Prove truth of (charge)".)