conducted with or marked by hidden aims or methods; 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

[ Syn: clandestine , hole-and-corner , hugger-mugger , hush-hush , secret , surreptitious , undercover , underground ]