Possible Clues:

• A letter reportedly used in acknowledgement
• Open declaration (last 3 + ...)
• Any of five letters read out is a confession
• Audibly, what could be you or I making assertion
• Assertion from one kind of character heard, found in either part of Twelfth Night
• Acknowledgement of a certain type of letter, we hear
• Statement that sounds like you perhaps
• Heartfelt assertion
• Profession
• Rule first cells must be set up for admission