I'm betting that it's the clutch, even though it doesn't appear to be slipping on the road.
It's amazing how a clutch will pull 600 pounds of motorcycle and rider down the road, but will slip when the engine is kicked over to start it, but it happens.
Usually, by the time a clutch is slipping on the kickstart, it's slipping on the road a bit too. The best test is getting ready to head up a hill in third or fourth gear at 3000 RPM or so, and open the throttle right up.
There's nothing else between the kickstart lever and the crankshaft that can "slip". Break, maybe, or jump several teeth, but not slip.
Lannis