Possible Clues:

• Sub on a plate
• *One may be six feet long
• Sub, and an apt description of 20-, 34-, or 42-Across
• Deli lunch ... or, based on a word hidden in 20-, 34- and 41-Across, what each of those answers is?
• Sub ... or a literal hint to 20-, 34- and 43-Across
• Sub
• Submarine (2 words)
• Deli offering
• Submarine