Authenticates or affirms to be true (7)
I believe the answer is:
attests
This clue is a double definition.
'authenticates' is the first definition.
(I've seen this in another clue)
'affirms to be true' is the second definition.
(I've seen this in another clue)
(Other definitions for attests that I've seen before include "Certifies" , "Sat test (anag.)" , "Confirms" , "Authenticates or affirms to be true" , "Gives evidence in court" .)