Related Clues:

• Mapmaker's aid
• Hint
• Starting point
• Indication
• One of 39-Across
• Fingerprint or dropped handkerchief, say
• Matrix
• This, for example
• Detective's need
• Print, maybe
• It may be found with a magnifying glass
• Guiding light?
• Inkling
• Fingerprint, perhaps
• Street plan, e.g.