形容词

1. 秘密的;密探的;卧底的
Undercover work involves secretly obtaining information for the government or the police.

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: clandestinecloak-and-daggerhole-and-corner(a)hugger-muggerhush-hushsecretsurreptitiousunderground