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Synonyms for Undercover (same or very similar meaning)

WordNet sense 1 (conducted with or marked by hidden aims or methods):
clandestine, cloak-and-dagger, hush-hush, secret, surreptitious, underground, hole-and-corner, hugger-mugger

WordNet sense 2 (secret or hidden; not openly practiced or engaged in or shown or avowed):
covert


From the ODE community, based on WordNetadd/edit