From Longman Dictionary of Contemporary Englishsecret agentˌsecret ˈagent noun [countable] PGPMsomeone whose job is to find out and report on the military and political secrets of other countries SYN spy
Examples from the Corpus
secret agent• Foley looked like anything in the world except a secret agent.• It was not every night, she reflected, that she dined with a secret agent.• Jack had specifically asked his secret agent to confine himself to a couple of current photographs and Polly's address.• They say that the better the secret agent, the less one hears about him.• The secret agent in his place, he wrote, the infiltrator safely ensconced. 