Main entry: secret, undercover, underground, surreptitious, hole-and-corner, hugger-mugger, hush-hush, clandestine, cloak-and-dagger
Definition: conducted with or marked by hidden aims or methods
Usage: clandestine intelligence operations; cloak-and-dagger activities behind enemy lines; hole-and-corner intrigue; secret missions; a secret agent; secret sales of arms; surreptitious mobilization of troops; an undercover investigation; underground resistance