Synonyms for Clandestine:

adj.

all (adjective)
hush-hush, secret, cloak-and-dagger, hole-and-corner, surreptitious, underground, undercover.
cloak-and-dagger (adjective)
hugger mugger.
latent (adjective)
hidden, unapparent, invisible, unrevealed, covert, camouflaged, unexposed, unobservable, concealed, veiled, latent, buried.
secret (adjective)
covert, furtive, arcane, classified, huggermugger, restricted, stealthy, hush-hush, secret, confidential.
secret, sly (adjective)
furtive, hidden, concealed, under wraps, private, covert, on the quiet, stealthy, hush-hush, underground, cloak-and-dagger, surreptitious, undercover.
surreptitious (adjective)
hole-and-corner.
underhand (adjective)
on the quiet.

n.

in strict confidence, mysterious, shadowy, secretive. huggermugger, show, sub rosa, under wraps. clandestine (noun)
hush-hush, on the quiet, cloak-and-dagger, covert, hugger mugger, surreptitious, underground, secret, undercover, hole-and-corner.
furtive (noun)
private, study at secret.

Other synonyms:

secretive, shadowy. Other relevant words:
on the quiet, private, hole-and-corner, underground, surreptitious, mysterious, shadowy, under wraps, secretive, huggermugger, cloak-and-dagger, undercover, hugger mugger.

Usage examples for clandestine

  1. He still clung to the hope of being able to effect his recapture; and to that end he employed- though in a clandestine manner- all the influence he could bring to his aid. – The White Gauntlet by Mayne Reid
  2. They served the same purpose to me as did the lamp of the lady called Hero to her lover Leander when he swam the Hellespont to pay her clandestine visits at night. – Allan and the Holy Flower by H. Rider Haggard
  3. Yes, but he does not know that Captain Butler has presumed- has dared to press a clandestine suit with you! – The Reckoning by Robert W. Chambers