Witness attending trial (6)
I believe the answer is:
attest
'witness' is the definition.
(thesaurus)
'attending trial' is the wordplay.
'attending' becomes 'at' (I've seen this in other clues).
'trial' becomes 'test' ('test' can be a synonym of 'trial').
'at'+'test'='ATTEST'
(Other definitions for attest that I've seen before include "Sign on" , "Vouch for" , "say, in court" , "Bear witness, declare to be true" , "Offer evidence" .)