(loosely) a person employed to collect the military secrets of one country and relay them to another, especially a person living in a foreign country while so working for his or her own nation; spy.
Origin
1830-40
Examples from the web for secret agent
That's why it's best to make sure your secret agent gear is top quality and working properly.
But if you were a secret agent working undercover, you would be extra careful too.
Behind every secret agent is an expert in acquiring high tech gadgetry.
The traditional peacetime source is the human secret agent.
The only possible plan of knowing what may be going on here would be to pay a secret agent.