有点复杂，但：

<?php

function get_single_term($post_id, $taxonomy) 
{
    $terms = wp_get_object_terms($post_id, $taxonomy);
    if(!is_wp_error($terms))
    {
        return \'<a href="\'.get_term_link($terms[0]->slug, $taxonomy).\'">\'.$terms[0]->name.\'</a>\';   
    }
}

//example
echo get_single_term(5, \'category\');