Untested but this might at least point you in the right direction.
HTML Code:
<a href="#" onclick="toggle('moreDescr');return false">see more</a>
<div id="moreDescr" style="display:none">
{$product.descr}
</div>
PHP Code:
{literal}
<script type="text/javascript">
function toggle(id) {
$("#"+id).toggle();
}
</script>
{/literal}
EDIT: now tested and adjusted