Testify (6)
'testify' is the definition.
(I've seen this in another clue)
This is all the clue.
'testify' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for witness that I've seen before include "Bystander" , "Spectator" , "It's news (anag.)" , "See (an event or crime)" , "Onlooker" .)