How to multi sort elements in a List

Your question is very unclear, but I suspect you want something like:

var okSORT = from element in unSORT
             orderby element.xValue descending, element.clusCode, element.DocCode
             select element;

That's assuming you want it sorted primarily by xValue (largest first), then by clusCode (lexicographically earliest first), then by DocCode (lexicographically earliest first). That will return an IEnumerable<item>. If you need a List<item>, you can just use the ToList() method:

// orderby clause broken into multiple lines to avoid horizontal scrolling - 
// that makes no difference
var okSORT = (from element in unSORT
              orderby element.xValue descending,
                      element.clusCode,
                      element.DocCode
              select element).ToList();

At that point it may make sense to use the extension methods directly:

var okSort = unSort.OrderByDescending(element => element.xValue)
                   .ThenBy(element => element.clusCode)
                   .ThenBy(element => element.DocCode)
                   .ToList();

Note that this does not sort the existing list in place. If you need to do that, you should create a Comparer<item> and use List<T>.Sort.

I would strongly suggest that you work on your naming, too. The type name, property names and variable names are all unconventional/unclear.