Synonyms : clandestine , hole-and-corner , hugger-mugger , hush-hush , secret , surreptitious , undercover , underground

Example Sentence

• 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