Take the stand at a trial (6)
I believe the answer is:
attest
'take the stand' is the definition.
(synonyms)
'at a trial' is the wordplay.
'a trial' becomes 'test' ('test' can be a synonym of 'trial').
'at'+'test'='ATTEST'
(Other definitions for attest that I've seen before include "show?" , "Verify" , "Corroborate" , "Give evidence - sounds a trial" , "Give testimony in a court of law" .)