It is Short Spindle for rear wheel drum sprocket - Part No. 580355
If its threads are stripped, take off the wheel to remove it and replace with a new one. As HOO has said the large nut may also be damaged, part no. 580359.
Just confirm these part numbers again with CMW when you order by phone.
It is likely that it was cross threaded when installed at some point, or over tightened.