Swear at trial (6)
I believe the answer is:
attest
'swear' is the definition.
(attesting is a kind of swearing)
'at trial' is the wordplay.
'trial' becomes 'test' (synonyms).
'at'+'test'='ATTEST'
(Other definitions for attest that I've seen before include "Give evidence in a court of law" , "Make evident" , "Give testimony in a court of law" , "Provide firm evidence" , "Demonstrate" .)