The Clear button allows end-users to clear an ASPxComboBox value, i.e., to set the editor value to null. Therefore, you don't need to create a null (blank) item. The button is displayed when the editor is not empty.
The Clear button availability is controlled by the DisplayMode property. By default, the property is set to Auto to display the button on touch devices only when the editor's DropDownStyle property is not set to DropDownList.