It could be the end of the piston misaligned on the 'fork' that operates the clutch. My old +8 (1983) also had a plastic clip that slides over the fork and helps to locate the piston on the fork.
The plastic part usually falls off and ens up in the bottom of the bellhousing. I had a few hours of fun finding it again and retrieving it using an 'endoscope' that I bought from Aldi.
