Question

I have the following. My issue is that when the user selects a client the “— Clients —” disappears from the combo box. Is emptyText the right property to be using here.

 {
                                xtype:'combobox',
                                name:'Clients',
                                id:'Clients',
                                displayField:'Name',
                                emptyText:'-------- Clients --------',
                                store:'Clients',
                                queryMode:'local',
                                mode:'local',
                                allowBlank:true,
                                valueField:'Id',
                                width:200
                            },

Answer 1

Short answer, ‘no’, the empty text is only designed to be a default value before anything is selected.

The suggestion to add to the store is correct, after you create your store and get your server side data, do a manual add for your dummy row (or alternatively ensure your server side dataset contains the dummy)

Something like:

var dummyModel = Ext.create('Namespace.MyModel', 
{
    id: -1,
    text: 'dummy text'
});
Ext.getStore('myStore').add(dummyModel)