Affirmed to be true (8)
I believe the answer is:
attested
'affirmed to be true' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for attested that I've seen before include "Declared to be true or genuine" , "Provided evidence" , "Provided proof" , "Declared that something was true" , "Affirmed by oath" .)