Possible Clues:

• Ticket stub, at times
• Drawer contained ticket
• Postponement ... or what was not performed in 17-, 31- and 48-Across?
• Ticket for a postponed ballgame
• Ticket from a postponed ball game
• Coupon for an out-of-stock item
• Redeemable ticket
• Invitation postponed when sewer inspection fails to start
• Examine said rule first to get ticket that's valid later
• Shower inspection may be postponed to later date