早速のお返事ありがとうございます。

サンプルのCheckBoxRendererみたいな感じで常時表示で行いたいのです。
ソースは下記の様にしております。

import mx.core.UIComponent
import mx.controls.ComboBox
class ComboBoxRenderer extends UIComponent
{
var Combo : MovieClip;
var listOwner : MovieClip;
var getCellIndex : Function;
var getDataLabel : Function;
var cards2 = [ {label:"test1", data:1}, {label:"test2", data:2}, {label:"test3", data:3} ];


function RadioCellRenderer()
{
}

function createChildren(Void) : Void
{
Combo = createClassObject(ComboBox, "ComboBox", 1, {styleName:this, owner:this});
Combo.addEventListener("click", this);
size();
}

calls size(), after setting
function size(Void) : Void
{
Combo.setSize(100, layoutHeight);
Combo._x = (layoutWidth-100)/2;
Combo._y = (layoutHeight-16)/2;
}

function setValue(str:String, item:Object, sel:Boolean) : Void
{
Combo._visible = (item!=undefined);
Combo.dataProvider = cards2;
Combo.editable="true";
Combo.getItemAt(1).label;

}

function getPreferredHeight(Void) : Number
{
return 16;
}

function getPreferredWidth(Void) : Number
{
return 100;
}

function click()
{
listOwner.editField(getCellIndex().itemIndex, getDataLabel(), Combo.selectedItem);
}

}