Hopefully, the metal you are seeing is from the threads of the screw and not the threads of the tonearm. If the latter, you can probably still salvage the tonearm. You may simply be able to use an oversize screw with a similar thread pattern and make new threads in the tonearm by screwing in the new screw. I would first try to achieve a satisfactory tension with the existing screw or one of the same size and thread pattern. If you can tighten the screw to a reasonable tension, I would leave well enough alone. If not, you could apply some automotive silicon to the threads and try to achieve a reasonable tension. You might even be able to create new threads with an automotive thread putty that cures to a fairly hard consistency. You would apply it to the screw just as you would the silicon and attempt to thread the screw back in the arm...waiting to tighten it until the putty cures. As a last resort, you might consider using some Loc-Tite. This stuff is used in automotive applications when you need the threads to bond to the female threads with considerable effort needed to break the seal once cured. I would only use that as a last resort and if I had no intentions of trying to remove the screw in the future.

It's toast if you have to drill out a screw on a tone arm.

Trust me. You can sell it on Ebay and get more than you originally paid for it. Broken.