You could achieve the desired scenario by modifying the default ControlTemplate in the RadAutoCompleteTextBoxStyle in order to include that button. Afterwards you will just need to implement the functionality to remove the selected item when that button is pressed and to hide it when there isn't any SelectedItem. I have prepared a sample project which demonstrates the exact approach (note that I have used Implicit Styles
in order to apply the theme).
Hope this helps.
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