Possible Clues:

• Superfluous individual
• Unwanted tagalong
• Based on the ends of 17-, 23-, 38- and 50-Across, unwanted thing that this puzzle lacks
• Superfluous participant
• Unnecessary tagalong (after 1-Across, 44-Across, 46-Across and 37-Down)
• One who might pull up a chair to a booth
• Extra person on a double date ... who's depicted by the last O in 17-, 24-, 39- and 52-Across?
• 20 percent of a spare tire?