Example : 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.

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