Related Clues:

• See 15-Across
• Friend or foe, e.g.
• It may be cracked
• Spy's secret
• Substantive
• Verb, for example
• Principles
• Pig Latin, e.g.
• Spy's writing, perhaps
• Sentence starter
• Subject, usually
• Something in writing?
• Secret message
• One if by land, two if by sea, etc.
• Value system