Observing secretly (6)
I believe the answer is:
spying
'observing secretly' is the definition.
('spy' can be a synonym of 'observe secretly')
This is the entire clue.
(Other definitions for spying that I've seen before include "collection of information" , "Catching sight of; agent's job" , "Keeping furtive watch" , "Acting as a secret agent" , "Engaging in espionage" .)