undercover

详细解释:a. 秘密从事的, 秘密的, 从事间谍活动的
【法】 暗中从事的, 秘密的

英语解释：

形容词 undercover:

conducted with or marked by hidden aims or methods同义词：clandestine, cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, secret, surreptitious, underground