undercover agent(n = noun.person) spy - (military) a secret agent hired by a state to obtain information about its enemies or by a business to obtain industrial secrets from competitors;
is a kind of intelligence agent, intelligence officer, operative, secret agent
has particulars: counterspy, mole, double agent, espionage agent, foreign agent, infiltrator, sleeper, margarete gertrud zelle, mata hari

N  warning, early warning, caution, caveat, notice, premonition, premonishment, prediction, contraindication, lesson, dehortation, admonition, monition, alarm, handwriting on the wall, mene mene tekel upharsin, red flag, yellow flag, fog-signal, foghorn, siren, monitor, warning voice, Cassandra, signs of the times, Mother Cary's chickens, stormy petrel, bird of ill omen, gathering clouds, clouds in the horizon, death watch, watchtower, beacon, signal post, lighthouse, sentinel, sentry, watch, watchman, watch and ward, watchdog, bandog, housedog, patrol, patrolman, vedette, picket, bivouac, scout, spy, spial, undercover agent, mole, plainclothesman, advanced guard, rear guard, lookout, cautiousness, monitor, guard camera, radar, AWACS, spy satellite, spy-in-the- sky, U2 plane, spy plane, warning, premonitory, monitory, cautionary, admonitory, admonitive, sematic, warned, forewarned, on one's guard, in terrorem, Int, beware!, ware!, take care!, look out!, fore (golf), mind what you are about!, take care what you are about!, mind!, ne reveillez pas le chat qui dort, don't wake a sleeping cat, foenum habet in cornu, caveat actor, le silence du people est la legon des rois, verbum sat sapienti, a word to the wise is sufficient, un averti en vaut deux.

activating agent, active agent, actor's agent, adhesive agent, agent, agent bank, agent orange, agent provocateur, agricultural agent, alkalating agent, alpha-adrenergic blocking agent, anaesthetic agent, anesthetic agent, antianxiety agent, antifungal agent, antimycotic agent, antipsychotic agent, antispasmodic agent, antiviral agent, ataractic agent, beta-adrenergic blocking agent, biological agent, blanching agent, bleaching agent, blistering agent, blocking agent, book agent, booking agent, bridge agent, business agent, causal agent, chelating agent, chemical agent, claim agent, cleansing agent, collection agent, county agent, customer agent, dissolving agent, double agent, drying agent, employment agent, espionage agent, estate agent, extension agent, fbi agent, federal agent, fixing agent, foreign agent, free agent, freight agent, general agent, government agent, house agent, hypoglycaemic agent, hypoglycemic agent, immunizing agent, indian agent, infectious agent, infective agent, inquiry agent, insurance agent, intelligence agent, internal revenue agent, land agent, law agent, lethal agent, literary agent, mill agent, mustard agent, narcotics agent, nerve agent, neuroleptic agent, neuromuscular blocking agent, organophosphate nerve agent, oxidizing agent, parliamentary agent, press agent, psychotropic agent, purchasing agent, real estate agent, reducing agent, road agent, secret agent, selling agent, shipping agent, solvating agent, special agent, station agent, surface-active agent, talent agent, theatrical agent, thrombolytic agent, ticket agent, toll agent, transfer agent, travel agent, undercover, undercover operation, undercover work, universal agent, wetting agent