Example : A furtive manner.
A sneak attack.
Stealthy footsteps.
A surreptitious glance at his watch.

Synonyms : furtive, sneak, sneaky, stealthy

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, undercover, underground