This article explains how to reset / clear RadComboBox.
Resetting the combobox depends on the AllowCustomText property - if it is set to True then you can clear the combo by calling its ClearSelection() server method and resetting its Text to an empty string:
combo.ClearSelection(); combo.Text = string.Empty;
If AllowCustomText is False - then the combo will always have one selected item (just like the standard DropDownList has). What you can do here is to select the first item (it can be the item holding the "Select a product" text for example):
combo.Items.Selected = true;
Additionally, you can clear the items by using clearItems() client-side method or combo.Items.Clear() server side method. Again, if the combobox's AllowCustomText is True then you need to manually set the Text of the RadComboBox to string.Empty.