If the clutch adjustment doesn't clear it up the problem could be with a contaminated clutch with oil or grease on the disk. This makes the clutch disk swell and not fully release. Clutches don't last forever, you may have to bite the bullet and replace the clutch assembly. Before you do anything drastic, check the trans fluid level. If the fluid is too low the synchros don't work as well and you get gear clash. Hope it's an easy fix for you.
