Possible Clues:

• Mailing address
• Numbered compartment for mail
• Many a bus. address, and a literal hint to 18-, 25-, 47- and 63-Across
• Popular address by party leader to opposition leader leads to a fight
• Where some cos. pick up their mail
• Abbr. on many letters (2 words)
• Street address alternative (Abbr.)
• Old boy restricted by disease — one gets lots of letters
• Alternative to a street address
• Certain mailing address, for short ... or a hint to 14 squares in this puzzle