(function($){
 
 //初始化清除按钮
 function initClear(target){
 var jq = $(target);
 var opts = jq.data('combo').options;
 var combo = jq.data('combo').combo;
 var arrow = combo.find('span.combo-arrow');
 
 var clear = arrow.siblings("span.combo-clear");
 if(clear.size()==0){
  //创建清除按钮。
  clear = $('<span class="combo-clear" style="height: 20px;"></span>');
  
  //清除按钮添加悬停效果。
  clear.unbind("mouseenter.combo mouseleave.combo").bind("mouseenter.combo mouseleave.combo",
  function(event){
   var isEnter = event.type=="mouseenter";
   clear[isEnter "combo-clear-hover");
  }
  );
  //清除按钮添加点击事件，清除当前选中值及隐藏选择面板。
  clear.unbind("click.combo").bind("click.combo",function(){
  jq.combo("setValue","").combo("setText","");
  jq.combo('hidePanel');
  });
  arrow.before(clear);
 };
 var input = combo.find("input.combo-text");
 input.outerWidth(input.outerWidth()-clear.outerWidth());
 
 opts.initClear = true;//已进行清除按钮初始化。
 }
 
 //扩展easyui combo添加清除当前值。
 var oldResize = $.fn.combo.methods.resize;
 $.extend($.fn.combo.methods,{
 initClear:function(jq){
  return jq.each(function(){
   initClear(this);
  });
 },
 resize:function(jq){
  //调用默认combo resize方法。
  var returnValue = oldResize.apply(this,arguments);
  var opts = jq.data("combo").options;
  if(opts.initClear){
  jq.combo("initClear",jq);
  }
  return returnValue;
 }
 });
}(jQuery));

<select id="myselect" panelHeight="auto" style="width: 102px;" editable="true" name="itcast">
 <option value="0">成都</option>
 <option value="1">传智播客</option>
 <option value="2">姚欣</option>
 <option value="3">小星星</option>
</select>

$('#myselect').combobox({
 required : true,
 editable : false
}).combobox("initClear");

.combo-clear {
 background-color: #E0ECFF;
 width: 18px;
 height: 20px;
 overflow: hidden;
 display: inline-block;
 vertical-align: top;
 cursor: pointer;
 opacity: 0.6;
 filter: alpha(opacity=60);
 background: url('images/combo_clear.png') no-repeat center center;
}
.combo-clear-hover {
 opacity: 1.0;
 filter: alpha(opacity=100);
 background-color: #eaf2ff;
}