I've got it too on my 95 M900. It could be either the large seal around the outside of the "driven" primary gear where it goes through the right side crankcase cover. Or could be the smaller seal that goes between the inner surface of the driven gear and the center hub of the clutch. Oil from either source gets slung to the same place - the outside perimeter of the clutch housing.
I've been waiting on the dealer to get a right side crankcase cover gasket. The other necessary parts are all there waiting on me. When I go into mine, I'm going to replace both seals mentioned above as normal good practice. To do it right, there is also a crankcase end bush seal to replace, an oil galley o-ring, and something else I can't remember without pulling out the Haynes manual. Something like seven seals, rings and gaskets total. I'm giving up on the case cover gasket and switching to silicone sealant as many others have done.
As long as the leak isn't bad, you can keep riding. Just clean out the excess oil occasionally (don't spray solvent cleaners on the clutch release bearing) and keep an eye out that the leak doesn't get bad enough to oil the back tire or lower the oil level below minimum.