WordNet (r) 3.0 (2006):

undercover
    adj 1: conducted with or marked by hidden aims or methods;
           "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"
           [syn: clandestine, cloak-and-dagger, hole-and-
           corner(a), hugger-mugger, hush-hush, secret,
           surreptitious, undercover, underground]

Moby Thesaurus II by Grady Ward, 1.0:

141 Moby Thesaurus words for "undercover":
   above water, abstruse, armored, at anchor, back-door, backstairs,
   beclouded, behind the curtain, behind the scenes, behind the veil,
   blind, buried, cased, ceiled, clandestine, cloaked, close, clouded,
   coated, concealed, confidential, coped, covered, covert, covertly,
   cowled, curtained, doggo, eclipsed, encapsulated, encapsuled,
   encased, enveloped, enwrapped, feline, filmed, floored, furtive,
   hid, hidden, hidden out, hidlings, high and dry, hole-and-corner,
   hooded, housed, hugger-mugger, hush-hush, in a cloud, in a corner,
   in a fog, in a whisper, in ambush, in darkness, in eclipse,
   in harbor, in hiding, in hidlings, in purdah, in safety, in secret,
   in the background, in the clear, in the dark, in the wings,
   incommunicado, latent, loricate, loricated, lurking, mantled,
   masked, muffled, mysterious, nobody the wiser, obfuscated, obscure,
   obscured, occult, occulted, on, on tiptoe, out of danger, packaged,
   past danger, paved, private, privy, prowling, pussyfoot,
   pussyfooted, quiet, recondite, roofed-in, screened, scummed,
   secluded, secluse, secret, secretly, sequestered, sheathed,
   shelled, shielded, shifty, shrouded, skulking, slinking, slinky,
   sly, sneaking, sneaky, sotto voce, stealing, stealthy, sub rosa,
   surreptitious, swathed, tented, terra firma, under an eclipse,
   under cover, under house arrest, under the breath, under the rose,
   under wraps, under-the-counter, under-the-table, underground,
   underhand, underhanded, underneath, unknown, unobtrusive, veiled,
   waiting concealed, walled, walled-in, with bated breath, wrapped,
   wrapped in clouds