The term is "cloaking", not "clocking".
Cloaking is a way of giving different content to visitors and search engines. This means that you can give real people a useful website, and search engines a keyword-stuffed page to con them into thinking your page does more than it really does.
Not surprisingly, search engines aren't too keen on this, and any sites they find using cloaking techniques are likely to be completely blocked. In other words, don't do it!