It sounds to me like a woodruff key is sheered, either on the big gear side or behind the motor sprocket. If I had to guess I would say it is under the motor sprocket, which isn't all that uncommon to happen, specially if your motor is running rough and jerky, or if you drop the clutch rapidly/often, or if the nut on the sprocket is a little loose. When you drop the clutch to start the motor it jerks that sprocket one way, when the motor pulls the chain it jerks it the other way. Too much jerking back and forth (and/or a loose nut on the sprocket) can cause that woodruff key to sheer.
There is a bar inside the clutch that links one side to the other (somewhere in this forum there is a post that shows a nice picture of a clutch all taken apart so you can see how it all fits together), if both woodruff keys are intact then it would have to be that bar that is bad, but I would definitely put my money on a woodruff key as the failure, probably under the drive sprocket. Try using a wider screwdriver on the screw to get it loose, you need to be able to remove that cover to do general maintenance......