Learn That Word

Synonyms for Undercover (same or very similar meaning)

WordNet sense 1 (conducted with or marked by hidden aims or methods):
clandestine, cloak-and-dagger, hush-hush, secret, surreptitious, underground, hole-and-corner, hugger-mugger

WordNet sense 2 (secret or hidden; not openly practiced or engaged in or shown or avowed):

From the ODE community, based on WordNetadd/edit