Try something like:
From the Internet..
WITH SET [TCat] AS TopCount([Product].[Subcategory].[Subcategory],10,[Measures].[Sales Amount]) MEMBER [Product].[Subcategory].[Other] AS Aggregate([Product].[Subcategory].[Subcategory] - TCat) SELECT { [Measures].[Sales Amount] } ON COLUMNS, TCat + [Other] ON ROWS FROM [Adventure Works]
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