Let me try to correct a couple of minor things some have suggested. First, if this is truly a clutch adjustment issue, the cable needs to be tightened, not loosened. The confusion may stem from adjustment at the perch end of the cable, and the fact that the adjuster is unscrewed(counter clockwise) to tighten the cable. Proper adjustment is critical, as the clutch must fully engage and disengage. If the cable is too tight, the clutch will not fully engage and it will slip. If it is too loose, it will not fully disengage and.... well, you are experiencing what can happen. Although TT is right- warped plates can also make this happen.
Then, I do not want to hijack this thread and start an oil thread. But I have been using the Honda synthetic oil WITH molybdenum for more than 20+ YEARS, and have never had a clutch issue. Or any other issue that could be attributed to oil. In fact, when obtaining a "new to me" SOHC4 bike, switching to this oil has solved minor shifting issues. That having been said, worn out oil (regardless of brand or type) can cause shifting issues. So, an oil change may help you, depending on how long it's been since the last one.