Witness watching over, perhaps? (6)
I believe the answer is:
attest
'witness' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
(Other definitions for attest that I've seen before include "Act as witness" , "Certify or bear witness" , "Give evidence in a court of law" , "Swear to" , "Sign on" .)