Check that the clutch cable isn’t beginning to seize; disconnect it at the trans and it should move relatively easy.
If it’s free, then it might be the bushing(s) in the transmission case for the lease shaft that are seizing. If so, it’ll likely mean pulling the trans; removing this shaft; cleaning or replacing these bushings.
Not a great diagram; it doesn't show the bushings but I hope it gives you an idea of what I'm talking about.
Dan.
Canadian "EH"