Related Clues:

• F or G, but not H
• In the lead
• Highly influential
• Start of a series
• Critically important
• Shrewd
• Simple
• Letter opener?
• End of the response
• Word in 101 course names
• Shift, e.g.
• Where the U.N. is in Manhattan, with 27-Across
• Place for a keystone
• Who's there
• Big wheel