Witness at a trial (6)

I believe the answer is:

attest

'witness' is the definition.
(I've seen this before)

'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" , "Act as witness" , "Confirm to be true" , "Vouch for" , "Give proof" .)

I've seen this clue in The Globe and Mail.

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