Confirm as genuine the act Auntie broadcast (12)
I believe the answer is:
authenticate
'confirm as genuine' is the definition.
(I've seen this before)
'the act auntie broadcast' is the wordplay.
'broadcast' indicates an anagram.
'the'+'act'+'auntie'='theactauntie'
'theactauntie' is an anagram of 'AUTHENTICATE'.
(Other definitions for authenticate that I've seen before include "Prove to be genuine" , "Show to be genuine" , "Validate" , "Confirm as genuine" , "Establish as valid" .)