Custom Post Types and Complex Content Hierarchy

You are trying to put Works and Clients on the same level and this is a little confusing. I would suggest you to keep the two entity separeted and the make all the relationship you need with the mentioned Posts to posts plugin.

First of all, create a custom type Work with his own slug ():

...
function create_post_type() 
{
    $labels = array( 
        'name' => 'Work',
        ...
    );
    $args = array(
        'labels' => $labels,
        'rewrite' => array('slug' => 'works'),
        ...
    ); 
    register_post_type('work',$args);
}
...

Then create the Clients:

...
function create_post_type() 
{
    $labels = array( 
        'name' => 'Clients',
        ...
    );
    $args = array(
        'labels' => $labels,
        'rewrite' => array('slug' => 'clients'),
        ...
    ); 
    register_post_type('client',$args);
}
...

Now install the Posts to posts plugin and define the relationship between the two entities:

function my_connection_types() {
    p2p_register_connection_type( array(
        'name' => 'works_to_clients',
        'from' => 'work',
        'to' => 'client'
    ) );
}
add_action( 'p2p_init', 'my_connection_types' );

At this point, your data structure is well built and you can move to the next stage (Clients and Works themes, breadcrumb, etc.)