A clamp is sure to damage the hub.
The easy way to do this is use some old spokes, cut the head (the hub end) off and make a double bend which can be inserted behind the freewheel. Find an old strong rim, and insert a few spokes in both directions, (one direction will do) on the gear side only, doesn't matter if they are loose as long as they will take the strain of turning the wheel.
Use your usual tool for removing the freewheel. Done this many times when hubs were brought in for a rebuild in this condition.
We used to carry spare spokes like this in case they were needed for a quick repair on the road.