Hi and thank you Fotis for your answer.
I have already an override. I am not an expert in programming. Which loop are you talking about?

In the editing options of the extrafield, I have to choose "text" if I want to add content. And the "text" category don't allow me to hide the tag in the frontend. Visitors will see the content of the tag and I only want this tag for changing the tag title.
I hope that you can understand my message.

Ok, it worked, but all the additional extrafileds are hidden now. I only want this extrafield hidden. My purpose it to change the tag title of the page from the backoffice. But in the front it has to be transparent for the visitors.
I'm gonna search again and maybe I will find a solution.
Thak you for your help.

This works well for displaying the extra field in Page Title

<?php
$newtitle= $this->item->extraFields->Naslov->value;
$document = JFactory::getDocument();
$document->setTitle($newtitle);
?>

but if the extra field is empty, the page link is displayed.

Is it possible to display the default page title if the extra field is empty?

sorry find solution:

<?php
$doc = JFactory::getDocument();
if ( isset( $this->item->extraFields->Naslov->value ) && ($this->item->extraFields->Naslov->value !=='') ) {
    $doc->setTitle('<title>'.$this->item->extraFields->Naslov->value.'</title>');
}

?>