add_filter('the_content', 'adsense_added_at_more_tag');
function adsense_added_at_more_tag($text) {
if( is_single() ) :
$ads_text = '
<div class="adsense">
тут что-то
</div>
';
$pos1 = strpos($text, '<span id="more-');
$pos2 = strpos($text, '</span>', $pos1);
$text1 = substr($text, 0, $pos2+7);
$text2 = substr($text, $pos2+7);
$text = $text1 . $ads_text . $text2;
endif;
return $text;
}