Possible Clues:

• Upper part of letter or card seen when sorted
• Upright and superior, for a man of letters
• Climber needs a sound transmitter
• Climber listened to a transmitter
• One rising in rank, e.g.
• Sounds like a person who goes to post a type of letter
• One rising
• It is exemplified in a letter of Lord Macaulay
• Part of letter from a person who posted it, say
• Climber needs a transmitter for radio communication