If it is actually the clutch that is slipping you might be able to adjust the air gap on the clutch to get some more life out of it. But if it has been slipping for awhile your best bet may still be to just replace it.
How Do I Adjust PTO Clutch on an Exmark Mower? | eHow.com
Before doing that though I would take a good look at your belts for cracks or glazing. If the belt looks OK make sure it is tight. I'm not sure how your mower is setup whether it uses a spring for belt back tension or has a mechanical adjustment. Might be the clutch is holding fine just the belts slipping.
How Do I Adjust PTO Clutch on an Exmark Mower? | eHow.com
Before doing that though I would take a good look at your belts for cracks or glazing. If the belt looks OK make sure it is tight. I'm not sure how your mower is setup whether it uses a spring for belt back tension or has a mechanical adjustment. Might be the clutch is holding fine just the belts slipping.