Englishundercover
adjective
they were arrested after a three-year undercover investigation:
covert, secret, clandestine, private, confidential, conspiratorial, underground, surreptitious, furtive, cloak-and-dagger, hole-and-corner, hugger-mugger, back-alley, backstair, stealthy, closet;
intelligence;
hidden, concealed, masked, veiled, shrouded;
Military black;
informal hush-hush, sneaky.
▷antonyms open, overt.