I've WordPress set up in a subdirectory and I would like one WordPress page to looks as if it's around the root.

Eg.
/wordpress/mypage

To look as:
/mypage

Presuming you are on Apache, plus you've got permalinks already working, you should use an .htaccess file inside your document root to rewrite yoursite.com/wordpress/mypage to yoursite.com/mypage.

If you do not curently have a .htaccess file inside your document root, create one and set this in:

RewriteEngine on

RewriteRule ^mypage$ wordpress/mypage

I am no .htaccess expert, but which should have the desired effect.