Bear witness during trial (6)
I believe the answer is:
attest
'bear witness' is the definition.
(I know that bear witness can be written as attest)
'during trial' is the wordplay.
'during' becomes 'at' (I can't justify this - if you can you should give a lot more credence to this answer).
'trial' becomes 'test' (synonyms).
'at'+'test'='ATTEST'
Can you help me to learn more?
(Other definitions for attest that I've seen before include "Sign on" , "Affirm by oath" , "Bear formal witness to, give evidence" , "Demonstrate" , "Vouch for" .)