1. (通常因违法而)偷偷摸摸的，暗地里的，秘密的
Something that is clandestine is hidden or kept secret, often because it is illegal.

1. conducted with or marked by hidden aims or methods

e.g. 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

Synonym: cloak-and-daggerhole-and-corner(a)hugger-muggerhush-hushsecretsurreptitiousundercoverunderground