Telford, part of the three-man crew, said he had no training in what to do if someone fell ill at the wheel.
He told Glasgow Sheriff Court that December 22 started as an ordinary day with the crew in "quite a joyful mood" because of the time of year, talking about Christmas and football.
They were chatting when he felt the vehicle veer to the left and turned to 58-year-old Clarke and shouted: "Harry, what are you ******* doing?" When there was no response he screamed: "You're killing people, Harry."
He said the driver's head was slumped to one side, and then his "whole body slumped to the left".
Telford added that "a bit of panic set in" as he screamed at and punched the driver to get a response.
The witness struggled to continue with his evidence as he described the truck hitting pedestrians.
When the vehicle stopped, he and his colleague Henry Toal got out and tried to help Clarke, who was unconscious, groaning and "grey". Five minutes later the driver asked what had happened.
Asked if he could have done anything to stop the truck, Telford said: "I don't think I could have done anything. Suppose I'm always going to be asking myself that question."
He added that he did not believe he could have reached the steering wheel or the brake without having to climb over the driver.
The court heard there were unopened bottles of beer in the truck's cab on the day of the crash. But Telford said he has no knowledge of them, it would be against council policy and no one had been drinking.
The FAI, expected to last around six weeks, was ordered after prosecutors said there was no evidence to warrant criminal proceedings.