without seeing it in person or a good video, i sounds like the clutch is bad, there should be fiber plates in between ,they can wear if the pack is allowed to loosten. Sometimes you can tighten the bolts that go through the springs,(usually about 6 ) tighten each bolt an the same amount of turns to keep pressure even.
Another way you can check to be sure it is the clutch pack (because it could be the drive chain jumped off), raise and block up the tiller so it cannot fall, shut tractor completely off and remove key, engage pto with engine off, now reach under the tiller and rotate the blades, if the one side of clutch pack rotates but does not turn the shaft half, it is the clutch. BE CAREFULL! DONT PINCH OR CRUSH YOURSELF. BE SURE TRACTOR IS OFF. AND BE SURE IMPLIMENT CANNOT FALL ON YOU. NEVER WORK ON PTO EQUIPMENT WHILE RUNNING OR IF TRACTOR CAN ACCIDENTALLY START.