How to removeField from within an Extension


I try to extend a SiteTree-Class and need to remove some fields.
I think that updateCMSFields gets called before the getCMSFields of the extended class.
So my removeField is not in affect…
But how to do it?

class OrderProductExtension extends Extension{
	public function updateCMSFields(FieldList $fields) {
			$fields->addFieldToTab('Root.Main',new LiteralField("Test","Test"));

Because I try to exntend another modul of mine I came up with adding a hook-point to my original class.

the updateCMSFields-Method which seems to be a build-in hook aswell seems to get inserted in the beginning of getCMSFields…

class Product extends Page{

        public function getCMSFields(){


             **$this->extend('addExtension', $fields);**

             return $fields;

class ProductExtension extends DataExtension{

         public function **addExtension(FieldList $fields)**{