The clutch friction disk may well be the culprit, and if the clutch has been slipping for other reasons, the friction disk is probably toast, but make sure the clutch is fully releasing. There is a screw type adjustment for it, but there may be some mechanical binding. If the linkage is in good shape and adjusted properly, some other things to look for are a broken pressure plate spring, and binding of the pressure plate against the axle. If there is a problem with the pressure plate or spring, it will have to come apart.
If the clutch does have to come apart, you might be able to rejuvenate the disc if it is just glazed and not too worn. If it is grooved, you might as well get a new disk, but before installing it, you should take some emery cloth and break the glaze on the pressure plates. Also, to prevent the pressure plate from binding against the driveshaft, put a little grease on the axle end to keep the pressure plate from binding (but don't get anything on the disk).