Have run into this, well not often, but as a tech, you remember those type jobs due to their expense, downtime, and customer gripes.
Pull the left hood side off. Remove the pin from the clutch linkage. There's a bar that runs down to the bellhousing/clutch release area, then it's connected to the link that runs back up to the steering console. With the pin out of the linkages, move the pedal-if it's still stiff, then your issue is in the pedestal mounted clutch pedal pivot shaft. If the pedal is free, the issue lies in the bellhousing of the tractor, which means time to split it, pull the release (throwout) bearing, and it's fork, off of the shaft, then remove and clean the corrosion from both the shaft and the holes that it rides in.
My Massey 1145 is having same issues, mine is in BOTH the linkages AND the clutch release. Yay. Another project.
The release bearing shaft is about 3/4" diameter. It wouldn't take much to drill the ends of it and then drill a tiny intersecting hole so that grease can get to the pivot points. BUT, I don't remember if doing so would even be accessible from under the tractor? If it's not accessible, it would be wasted effort.