It's a masculine adjective, and the feminine form is "clandestine".
Here are some examples:
* Une réunion clandestine: A clandestine meeting.
* Une relation clandestine: A clandestine relationship.
* Des opérations clandestines: Clandestine operations.
"Clandestin" is a direct borrowing from the Latin "clandestinus", which itself derives from "clandestinus", meaning "secret" or "hidden".