The Collaborative International Dictionary of English v.0.48:

Clandestine \Clan*des"tine\, a. [L. clandestinus, fr. clam
   secretly; akin to celare, E. conceal: cf. F. clandestin.]
   Conducted with secrecy; withdrawn from public notice, usually
   for an evil purpose; kept secret; hidden; private; underhand;
   as, a clandestine marriage. --Locke.

   Syn: Hidden; secret; private; concealed; underhand; sly;
        stealthy; surreptitious; furtive; fraudulent. --
        Clan*des"tine*ly, adv. -- Clan*des"tine*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

clandestine
    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:

26 Moby Thesaurus words for "clandestine":
   back-door, backstairs, covert, feline, furtive, hidlings,
   hole-and-corner, hugger-mugger, privy, quiet, shifty, skulking,
   slinking, slinky, sly, sneaking, sneaky, stealthy, surreptitious,
   under-the-counter, under-the-table, undercover, underground,
   underhand, underhanded, unobtrusive



Bouvier's Law Dictionary, Revised 6th Ed (1856):

CLANDESTINE. That which is done in secret and contrary to law.
     2.Generally a clandestine act in case of the limitation of actions will
prevent the act from running. A clandestine marriage is one which has been
contracted without the form which the law has prescribed for this important
contract. Alis. Princ. 543