Override the 'inputEvents' property to set the key handler for the combobox.
$('#cb').combobox({
data: data,
valueField: 'id',
textField: 'text',
editable: false,
inputEvents: $.extend({}, $.fn.combobox.defaults.inputEvents, {
keyup: function (e) {
if (e.which == 46) { // press Delete key
$('#cb').combobox('clear')
}
}
}),
})