Bear witness at a trial (6)
I believe the answer is:
attest
'bear witness' is the definition.
(synonyms)
'at a trial' is the wordplay.
'a trial' becomes 'test' ('test' can be a synonym of 'trial').
'at'+'test'='ATTEST'
(Other definitions for attest that I've seen before include "Give evidence - sounds a trial" , "Declare to be true or genuine" , "Adjure" , "say, in court" , "Act as witness" .)