clandestine
Meaning: secret
Usage: The secret agent was involved in clandestine spying activities.
surreptitious
Meaning: stealthy
Usage: The surreptitious CIA agents were able to get in and out of the house without anyone noticing.
covert
Meaning: secretly engaged in
Usage: Nerwin waged a covert campaign against his enemies, while outwardly appearing to remain friendly.
arcane
Meaning: obscure, secret, known only by a few
Usage: The professor is an expert in arcane Lithuanian literature.
obscure
Meaning: unclear, partially hidden
Usage: Because he was standing in the shadows, his features were obscure.