cloak-and-dagger
1. [ adjective ] conducted with or marked by hidden aims or methods
Synonyms: secret clandestine undercover underground on_the_quiet hush-hush surreptitious hugger-mugger hole-and-corner
Examples:

"clandestine intelligence operations" "cloak-and-dagger activities" "secret missions" "surreptitious mobilization of troops" "an undercover investigation" "underground resistance"

Related terms: covert