Possible Clues:

• 'The . . . . . . . . . broke the camel's back'
• Learner botching start was the ultimate irritation
• False starts breaking rule? One too many perhaps
• Back breaker?
• Cause for exclaiming 'That's it!'
• 'I've had it!' inspiration
• 'It is the ..... that breaks the camel's back'
• Endure Jack injuring camel
• Start laws (anag) ? breaking point?
• Endures skin complaint arising - too much to bear?