undercover agent
— Noun
– English
～ (military) a secret agent hired by a state to obtain information about its enemies or by a business to obtain industrial secrets from competitors
undercover operation
— Noun
– English
～ an operation involving secret work within a community or institution
undercover
— Adjective
– English
～ 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"