Related Clues:

• Letters on letters
• ''Quickly!'' acronym
• Word on an urgent message
• Without delay, in a memo
• Abbr. on 14-Across
• 'Right away!'
• ''On the double!''
• Posthaste, briefly
• Abbr. on a business letter
• Memo header
• Letters akin to P.D.Q.
• 'Stat'
• Abbr. at the top of a memo
• In very short order
• Express letters?