Prove aunt is best at rambling (12)
I believe the answer is:
substantiate
'prove' is the definition.
(thesaurus)
'aunt is best at rambling' is the wordplay.
'rambling' indicates anagramming the letters.
'aunt'+'is'+'best'+'at'='auntisbestat'
'auntisbestat' anagrammed gives 'SUBSTANTIATE'.
(Other definitions for substantiate that I've seen before include "Confirm" , "Prove truth of (charge)" , "Prove to be true" .)