clandestine (adj.)

• backstairs
• covert
• feline
• furtive
• hole-and-corner
• hugger-mugger
• privy
• quiet
• shifty
• sly
• sneaking
• sneaky
• stealthy
• surreptitious
• under-the-counter
• under-the-table
• undercover
• underground
• underhand
• underhanded
• unobtrusive