Over center clutch spring.

It ain't easy. I stretched it out with my floor jack and added small finishing nails between the coils on both sides until I had adequate clearance. Then put it in place and pry the nails out one by one (you still might have to pry each of the coils a bit with a screwdriver or small pry bar to get the nails). Be careful not to stretch the spring too far or you will need a new one. One guess as to how I know that.