When WP_Query only returns 1 post, how do I automatically redirect to that post?

You can do this by checking the number of results returned by your query. Try this.

<?php

    $rawfiltertag = get_post_meta( $post->ID, '_cmb_client_name', true );
    $filtertag = sanitize_title( $rawfiltertag );

    // wp_query arguments
    $args = array (
        'post_type' => 'post',
        'meta_query' => array(
            array(
                'key' => '_cmb_client_target',
                'value' => $rawfiltertag
            )
        )
    );

    // the query
    $the_query = new WP_Query( $args );

    if ( $the_query->post_count == 1 && $the_query->max_num_pages == 1 ) {
        wp_redirect( get_permalink( $the_query->posts['0']->ID ) );
        exit;
    }

?>