A tense trial witness (6)

I believe the answer is:

attest

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

'a tense trial' is the wordplay.
'tense' becomes 't' (grammatical abbreviation).
'trial' becomes 'test' (synonyms).
'a'+'t'+'test'='ATTEST'

(Other definitions for attest that I've seen before include "Give evidence - sounds a trial" , "Act as witness" , "Affirm by oath" , "Provide proof of, declare to be true (6)" , "Offer evidence" .)

I've seen this clue in The Times and The Sunday Times.

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