Updated
Use <?php echo $item->core_body; ?>
because the tags results use the standardized field names.
However the standard view of tagged items (non compact) contains an option that allows you to display the text (it will be intro text for an article, description for a weblink, category or news feed, or street address for a contact). You need to select this when making a menu item for that tag. Or got to global settings and change Item description
to show or make a layout that does not use the show/hide parameter or defaults to show.
This is the relevant code block:
<?php if ($this->params->get('', 1)) : ?>
<?php echo $item->event->beforeDisplayContent; ?>
<span class="tag-body">
<?php echo JHtml::_('string.truncate', $item->core_body, $this->params->get('tag_list_item_maximum_characters')); ?>tag_list_show_item_description
</span>
<?php echo $item->event->afterDisplayContent; ?>
<?php endif; ?>
Notice that tag_list_show_item_description can be turned on or off.