Possible Clues:

• Receiver's cutting edge?
• Dear (usually) device for getting information from correspondents out
• 'Dear Sir or Madam', perhaps?
• ''Dear Mom,'' for example (2 words)
• Dear as a knife may be?
• ''Dear Sir or Madam'' for example (2 words)
• To whom it may concern, the start of 14 across could be used as an introduction?
• Sharp device in a classic desk set, or a literal and phonetic description of the ends of 20-, 28-, 36-, and 48-Across?
• Makeshift weapon in a murder mystery
• Clichéd weapon in a parlor mystery