See a secret agent (3)
I believe the answer is:
spy
This clue is a double definition.
'see' is the first definition.
(spy can mean to see or observe)
'a secret agent' is the second definition.
(spy is a kind of secret agent)
(Other definitions for spy that I've seen before include "Catch sight of" , "Keep watch secretly, maybe for foreign government" , "Undercover agent" , "Mole, agent" , "Discover" .)