Possible Clues:

• Mark nothing for writer in hospital, as that's a job for snoop perhaps
• Tool used on an envelope, or the first word of 15-, 25- or 60-Across?
• "Dear Mom," for example (2 words)
• Implement for tearing envelopes
• Dear Sir or Madam, perhaps?
• Dear sir, it is a paper knife
• Office snoop?
• Device used on incoming mail
• Paper knife
• Machine for dealing with the post