Done in a private place or way; done secretly.
1) I took a clandestine peek at the tag of a diamond necklace.
2) The group held a clandestine meeting.
Middle French or Latin,; Middle French clandestin, from Latin clandestinus, from clam which means 'secretly'.
backstairs, behind-the scenes, secret, covert, furtive, hole-and-corner.
open, overt, public.