Are you talking about the clutch knob itself or the screw that is on the clutch knob so you don't unscrew the knurled part all the way off when you want to wind the bobbin?
It may take some time to get it off. Do you have Tri-Flow? I know just a drop around the edge let set overnight has helped some stubborn screws I've had. I have also used Kroil and some other things. Maybe position the machine so it will seep down to the screw threads. Heat from a blow dryer might help, too.
You might be able to use a strap wrench for a stuck knob, once the screw is out. It is a different kind of knob, but John made a wrench for the Atlas machine at
Pink Atlas - AAZ-60 - what cams?? You'll notice a piece of rubbery strip that helps to grab and protect the finish.
Janey - Neat people never make the exciting discoveries I do.