Possible Clues:

• Alternative strategy, or a hint to the word whose letters progress in reverse through 18-, 23-, 36-, 49- and 59-Across
• "Just in case" contingency
• Safety net, of a sort, and a literal feature of 17-, 23-, 39- and 52-Across
• Safety option
• "Just in case" strategy
• 'Just in case' strategy
• It's often labeled 'B'
• It's often labeled "B"
• Alternate strategy
• "Just in case" strategy, and a hint to a hidden letter sequence in 17-, 24-, 37- and 52-Across