function define_custom_excerpt_metabox( $options ) {
    $announcement_custom_metabox = new_cmb2_box( array(
       'id' => 'custom_meta',
       'title' => __( 'Announcement Custom Excerpt', 'text-domain' ),
       'object_types' => array( 'te_announcements' ), // Post type
       'context' => 'advanced',
       'priority' => 'high',
       'show_names' => true, // Show field names on the left
    ) );
    // Container class
    $announcement_custom_metabox->add_field( array(
       'name' => __( 'Custom Excerpt', 'text-domain' ),
       'desc' => __( 'Enter the custom excerpt for this announcement in the field above.', 'text-domain' ),
       'id' => 'announcement_custom_excerpt',
       'type' => 'wysiwyg',
    ) );
}
add_action( 'timeline_express_metaboxes', 'define_custom_excerpt_metabox' );

function replace_default_timeline_express_excerpt( $excerpt, $post_id ) {
    if ( timeline_express_get_custom_meta( $post_id, 'announcement_custom_excerpt', true ) ) {
       return apply_filters( 'the_content', get_post_meta( $post_id, 'announcement_custom_excerpt', true ) );
    } else {
       return $excerpt;
    }
}
add_filter( 'timeline_express_frontend_excerpt', 'replace_default_timeline_express_excerpt', 10, 2 );