To exclude a directory and all virtual requests to it, extend the regular rules:
# WordPress
# Images, Stylesheets etc. don't need a HTML 404
RewriteCond %{REQUEST_URI} !.+\.\w{2,4}$
# Existing file
RewriteCond %{REQUEST_FILENAME} !-f
# Existing directory
RewriteCond %{REQUEST_FILENAME} !-d
# Symbolic link
RewriteCond %{REQUEST_FILENAME} !-l
# Do not touch the directory 'clients'
RewriteCond %{REQUEST_URI} ! ^/clients/.*
RewriteRule ^ index.php [L]