There may be other or simple way to make it works but here I write code for you and it's tested. Just place below two functions into your theme files and you done.
function get_first_tag()
{
// to check whether this is question page
if($this->template == 'question') {
//get current url for postid
$myurl=$this->request;
$myurlpieces = explode("/", $myurl);
$pid=$myurlpieces[0];
//run query to get post tags
$query = "SELECT tags FROM ^posts WHERE postid=$pid"; //run query
$tags = qa_db_query_sub($query);
//loop through
while($tag=qa_db_read_one_assoc($tags, true)){
$tag = explode(',', $tag['tags']); //convert tag items into array
return array_shift($tag) . ' | '; // this will get first tag from array
}
}
}
==============================================================
function head_title()
{
$pagetitle=strlen($this->request) ? strip_tags(@$this->content['title']) : '';
$headtitle=(strlen($pagetitle) ? ($pagetitle.' - ') : '').$this->content['site_title'];
// add tags function here
$this->output('<TITLE>'. $this->get_first_tag() . $headtitle.'</TITLE>');
}