Hmmm...
This will give you what you need. Add to editpost_start.
PHP Code:
$getinfo = $db->query_read("SELECT fieldname FROM " . TABLE_PREFIX . "post WHERE postid = " . $postinfo['postid'] . "");
$fieldinfo = $db->fetch_array($getinfo);
$fieldname = $fieldinfo['fieldname'];
To have this working, you must set your field in your edit post template as <input type="text" value="$fieldname" name="fieldname" *>