Espionage (6)
I believe the answer is:
spying
'espionage' is the definition.
(I know that espionage can be written as spying)
This is all the clue.
(Other definitions for spying that I've seen before include "Keeping furtive watch" , "Acting as a secret agent" , "Activity undertaken by Blunt and Philby" , "Catching sight of; agent's job" , "Engaging in espionage" .)