Related Clues:

• Piano part
• One of 39-Across
• Fingerprint or dropped handkerchief, say
• Map feature
• This, for example
• Detective's need
• A major, maybe
• Opener
• F or G, but not H
• Print, maybe
• It may be found with a magnifying glass
• Primary
• Critically important
• Guiding light?
• Inkling