Confirm a time to be on trial (6)
I believe the answer is:
attest
'confirm' is the definition.
(thesaurus)
'a time to be on trial' is the wordplay.
'time' becomes 't'.
'to be on' is a charade indicator (letters next to each other).
'trial' becomes 'test' ('test' can be a synonym of 'trial').
'a'+'t'+'test'='ATTEST'
(Other definitions for attest that I've seen before include "Confirm to be true" , "Declare (that something is the case)" , "Give evidence in a court of law" , "Affirm by oath" , "Offer evidence" .)