conducted with or marked by hidden aims or methods
e.g. clandestine intelligence operations
e.g. cloak-and-dagger activities behind enemy lines
e.g. hole-and-corner intrigue
e.g. secret missions
e.g. a secret agent
e.g. secret sales of arms
e.g. surreptitious mobilization of troops
e.g. an undercover investigation
e.g. underground resistance
Synonym: clandestine cloak-and-dagger hole-and-corner(a) hugger-mugger secret surreptitious undercover underground