You can retrieve the combobox's text first and then restore its text after calling 'setValue' method.
var cc = $('#cc');
var text = cc.combobox('getText');
cc.combobox('setValue', yourvalue');
cc.combobox('setText', text'); // restore the original text
Another way to set the value only is to call 'setValue' method of combo instead.
var cc = $('#cc'); // the combobox object
cc.combo('setValue', yourvalue);