Ans:

CityComboBox.as
package components
{ 
import mx.controls.ComboBox;
public class CityComboBox extends ComboBox
{
public function CityComboBox()
{
dataProvider = [ "Delhi", "Mumbai", "Chennai", "Kolkata" ];
}
}
}

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
               xmlns:s="library://ns.adobe.com/flex/spark"
               xmlns:mx="library://ns.adobe.com/flex/mx">
<custom:CountryComboBox />
</s:Application>