[cflsystems]

You just have to put the field value in an <a> tag. Look in skin1/modules/Extra_Fields/product.tpl

You can replace
<td class="property-value">{$v.field_value}</td>
with
<td class="property-value">
{if $v.field eq "yyyy"}
<a href="{$v.field_value}">xxxx</a>
{else}
{$v.field_value}
{/if}
</td>