Witnessing (6)
'witnessing' is the definition.
('see' can be a synonym of 'witness')
This is all the clue.
(Other definitions for seeing that I've seen before include "Meeting" , "Viewing" , "since" , "... is believing, it's said" , "Noticing" .)
(Other definitions for onlooking that I've seen before include "Observer is" , "Observing" .)