Solution:
RadDateTimeEditor
dte = new RadDateTimeEditor();
((
RadDateTimeEditorElement)((RadDateTimeEditor)dte).EditorElement).Opened += new EventHandler(SelectDateRange_Opened);
private
void SelectDateRange_Opened(object sender, EventArgs e)
{
RadDateTimePickerBehaviorDirector
behavior = ((RadDateTimePickerElement)(sender)).GetCurrentBehavior();
((
RadDateTimePickerCalendar)behavior).Calendar.ClearButton.Visibility = ElementVisibility.Hidden;
}