The 'x' icon that you are referring to is coming from the IE browser and is not related to RadMonthYearPicker. You can place simple input element on a page and you will still notice the same 'x' icon for clearing the input. Some more details in this matter could be found in the following forum thread:
As for creating such icon within an input element in other browser, you can take a look at this forum thread:
Finally, if you need to have such icon outside of an input, you can use a simple ImageButton with a custom 'x' icon:
Hope this helps.
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