Change your support field to this
'supports' => array( 'title', 'editor', 'author', 'thumbnail', 'excerpt', 'comments' ) );
Eg:
<?php
add_action( 'init', 'create_testimonial_posttype' );
function create_testimonial_posttype(){
register_post_type( 'testimonials',
array(
'labels' => array(
'name' => __( 'Testimonials' ),
'singular_name' => __( 'Testimonial' )
),
'public' => true,
'has_archive' => true,
'rewrite' => array('slug' => 'clients'),
'supports' => array('title','thumbnail','editor','page-attributes','excerpt'),
)
);
}
?>