If the PTO lever is in neutral, the clutch should not have an effect. The blade carrier on a brush mower will run it for a long...long time (there is no brake on these PTOs). Does the mower start without the lever being in gear when you start the tractor? If you start the tractor with the mower attached, and the PTO in neutral, and your foot on the clutch all the way does the mower start to turn? does it start to turn when you let the foot off the clutch even with the lever in neutral? If you step on the clutch fully with the PTO lever engaged, does the mower start to coast to a stop?