Related Clues:

• Kept hidden
• Prefix meaning ''inner''
• Opposite of ''exo''
• Prefix denoting inner
• Opposite of "ecto"
• Known by only a few
• Undercover
• Something given, often not to be passed on
• Interior: Prefix
• Whispered thing
• Internal: Prefix
• Hidden, as eight related pieces in the grid
• Prefix denoting within
• For only a select few
• Prefix with -morphism