This is most likely a theme based question, a link to your site and what your active theme are would be useful for someone to offer any suggestions.
Line 84 of the header.php template file in a non-edited version of iDream reads like this:
wp_list_pages('title_li=&depth=0&sort_column=menu_order&child_of=0');
Edit it to read this:
wp_list_pages('title_li=&depth=0&sort_column=menu_order&child_of=0&exclude=90');
This will specifically exclude the page with ID=90 (Clients page) from the navigation menu.
You can alternatively use CSS to hide the link as well … see this article I wrote some time ago: http://buynowshop.com/2009/07/hide-a-menu-page/
Hopefully this will get you looking in the direction you want to go.
