Give evidence during trial (6)
I believe the answer is:
attest
'give evidence' is the definition.
(thesaurus)
'during trial' is the wordplay.
'during' becomes 'at' (I can't justify this - if you can you should believe this answer much more).
'trial' becomes 'test' ('test' can be a synonym of 'trial').
'at'+'test'='ATTEST'
Can you help me to learn more?
(Other definitions for attest that I've seen before include "Bear formal witness to, give evidence" , "Confirm to be true" , "say, in court" , "show?" , "Declare (that something is the case)" .)