Using $wpdb object in a widget

If you really just want a single row, use $wpdb->get_row($sql) instead.

But your query looks funny…it looks like you’re trying to get the latest ‘client status’ for the user in which case your query should be:
'SELECT status FROM Clients WHERE UserID = '.$current_user->ID.' ORDER BY ID DESC LIMIT 1'.

And if you do that, you can then even use $status = $wpdb->get_var($sql);.

Lastly…did you consider using add_user_meta/get_user_meta instead? There’s a lot of benefits to using the native tables and functions, such as that the records get cleaned up when you delete the user and the likes.