secret agent 
noun
A spy acting for a country.At the same time, they were, and still are, fascinated by the exploits of secret agents and counterspies....
- For the longest time, we were used to our spies being secret agents.
- Did I cut the figure of an imposing secret agent of the intelligence field then?
Synonyms
spy, agent, double agent, counterspy, field agent, undercover agent, operative, plant, infiltrator, mole
North American informal spook
archaic intelligencer