Bear witness at trial (6)
I believe the answer is:
attest
'bear witness' is the definition.
(I know that bear witness can be written as attest)
'at trial' is the wordplay.
'trial' becomes 'test' (synonyms).
'at'+'test'='ATTEST'
(Other definitions for attest that I've seen before include "Confirm to be true" , "Adjure" , "Demonstrate" , "Provide firm evidence" , "Vouch for" .)