Bear witness at the experiment (6)

I believe the answer is:

attest

'bear witness' is the definition.
('attest' can be a synonym of 'bear witness')

'at the experiment' is the wordplay.
'the experiment' becomes 'test' (test is a kind of experiment).
'at'+'test'='ATTEST'

(Other definitions for attest that I've seen before include "Offer evidence" , "Make evident" , "Corroborate" , "Affirm by oath" , "Swear to" .)

I've seen this clue in the Evening Standard.

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