Possible Clues:

• Is it the final fodder that causes the finish to the camel?
• The . . . . . . . . . broke the camel's back'
• Something that finally makes a situation intolerable
• The final bit of thatch is the finisher
• What finally causes breakdown in agitated stalwarts
• Final patience-tester
• Backbreaking event?
• Unbearable part of the camel's load
• Something tipping the balance
• Bringing up the rear, stalks camel's killer