You have to do two things, one, make sure there is some "play" in the clutch lever. Second, remove the cover on the left side. There is a rod with threads and a nut. There should be play in this also. If it is tight, your problem may be in the push rod, not in the clutch lever on the handlebar. Both are easy to do. The rod should just have a little play. If it doesn't, the effect is the clutch plates do not contact each other fully, causing the clutch to slip. If the clutch lever returns all the way (no play) then that can cause the clutch to slip also. Do the push rod first, with slack in the clutch lever, then do the clutch lever. The clutch lever should have about 3/4 inch play at the outer end of the lever.