Clandestine describes something done secretly, or in a private place or way.

// The wedding was a clandestine affair in Las Vegas.

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"On the surface, it uses the traditional tropes of the spy movie—a secret intelligence network, cryptic codenames, clandestine meetings in public places—but Ghost Trail isn’t exactly thrilling, certainly not in the manner of a John le Carré novel." — Damon Wise, Deadline, 15 May 2024

Psst!—if your first instinct, upon being asked what you’ve been up to, is to clam up, your querier may suspect you’ve been involved in some clandestine activities. Clandestine often substitutes for secret and covert, and it is commonly applied to actions that involve secrecy maintained for an evil, illicit, or unauthorized purpose, as in "clandestine activities pursued under cover of night." It comes to English by way of Middle French, from the Latin word clandestinus, which is itself from the Latin adverb clam, meaning "secretly." Note that this clam is not the ancestor of the English word clam, despite how tightly sealed and thus secretive the bivalves may seem.