Basically I'm asking for [`optionLabel` from `DropDownList`][1]. I can't do it using the datasource, because my data is remote. [1]: http://docs.telerik.com/kendo-ui/api/javascript/ui/dropdownlist#configuration-optionLabel
This indeed is not supported, and we don't have plans to provide built-in support for this behavior. Feel free to use a pseudo placeholder item for the combobox and implement your own solution when selecting it.
Actually what I need is, the user can select the placeholder/empty option back which will make the value of the combo box undefined.
For the combobox you have the placeholder option (http://docs.telerik.com/kendo-ui/api/javascript/ui/combobox#configuration-placeholder) which serves the same purpose as the optionLabel in the dropdown list.