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