How to put X inside textfield to clear text in extjs

You have to use a Ext.form.field.Trigger. Here is a example for that

Ext.define('Ext.ux.CustomTrigger', {
    extend: 'Ext.form.field.Trigger',
    alias: 'widget.customtrigger',
    initComponent: function () {
        var me = this;

        me.triggerCls = 'x-form-clear-trigger'; // native ExtJS class & icon

        me.callParent(arguments);
    },
    // override onTriggerClick
    onTriggerClick: function() {
        this.setRawValue('');
    }
});

Ext.create('Ext.form.FormPanel', {
    title: 'Form with TriggerField',
    bodyPadding: 5,
    width: 350,
    renderTo: Ext.getBody(),
    items:[{
        xtype: 'customtrigger',
        fieldLabel: 'Sample Trigger',
        emptyText: 'click the trigger'
    }]
});

For ease of testing, here is a JSFiddle

This is what works for me with the CSS:

CSS

    .x-form-clear {
        background-image: url('../../resources/themes/images/default/form/clear-trigger.gif');
        background-position: 0 0;
        width: 17px;
        height: 22px;
        border-bottom: 1px solid #b5b8c8;
        cursor: pointer;
        cursor: hand;
        overflow: hidden;
    }

    .x-form-clear-over {
        background-position: -17px 0;
        border-bottom-color: #7eadd9;
    }

    .x-form-clear-click {
        background-position: -68px 0;
        border-bottom-color: #737b8c;
    }

Class

Ext.define('Ext.ux.form.field.Clear', {
    extend: 'Ext.form.field.Trigger',

    alias: 'widget.clearfield',

    triggerBaseCls: 'x-form-clear',

    onTriggerClick: function() {
        this.setValue();
    }
});

Usage

Ext.create('Ext.container.Container', {
    renderTo: Ext.getBody(),
    items: [
        Ext.create('Ext.ux.form.field.Clear', {
            fieldLabel: 'Clear Field',
            cls: 'clear-trigger'
        })
    ]
})

Or use this 'clearbutton' plugin: http://www.eekboom.de/ClearButton.html

I like it because it's just a plugin, one line, instead of requiring a custom subclass.

Also, it can be used on all kinds of fields, not just on a textfield.