Clutch not engaging - help

#1
Hi,

I bought my son the Baja 165 last year and we seem to have a new problem with getting the rear wheel to spin. I pulled the clutch plastic cover off and can verify that the first chain is spinning properly. The issue is that where the chains come together at a pair of sprockets...the gear used to drive the rear wheel chain does not move...the front sprocket spins like crazy but for some reason, whatever engages the rear sprocket appears broken. There is a large gold color nut that appears to fasten the one sprocket on top of the other and I have tightened it but it comes loose immediately.

Any ideas?

Thanks
 
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