I haven't see one like that before, but I agree it seems to be screwed on. The locknut would have no purpose if it were pressed on.
it looks to me as if you need to unscrew the locknut until it contacts the shoulder on the crankshaft, so you can use a spanner on the nut to hold the crankshaft while you screw off the clutch. However I can't be certain, so I suggest you proceed reasonably gently. There does not seem to be rust on the crankshaft thread, so hopefully the clutch will just unscrew from it easily.
