Possible Clues:

• Tense construction worker set off
• Trotter's first fiddle over 25 nets £1,000 for his friend?
• He endeavours to sit astride horse and set off
• Activate theatre's opening with theatrical employee
• Gun-firing device
• Square with man working on boat to set in motion
• Activating device
• Set off in time ahead of scaffolder
• Revolutionary surrounded by equipment wanting a cause
• Set off with Pooh's friend round river