WordNet (r) 3.0 (2006):

secret agent
    n 1: a person secretly employed in espionage for a government
         [syn: secret agent, intelligence officer, intelligence
         agent, operative]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "secret agent":
   CIA man, asker, catechist, cloak-and-dagger operative,
   counterintelligence agent, counterspy, cross-interrogator,
   cross-questioner, detective, double agent, espionage agent,
   examiner, inquirer, inquisitionist, inquisitor, inside man,
   intelligence agent, interlocutor, interpellator, interrogator,
   interrogatrix, interviewer, military-intelligence man,
   naval-intelligence man, operative, opinion-sampler, poller,
   pollster, prober, querier, querist, questioner, questionist,
   quizzer, reconnoiterer, sampler, scout, spotter, spy, spy-catcher,
   undercover man