jQuery, JS or CSS...
CSS route is only going to work for some visitors since it would require :focus which isn't supported by all browsers ...
jQuery would be an option, but if you're not doing any other jQuery on the page you'll be loading the library for the sake on one small effect.
JS would proberly be the best route, attaching an onclick event that removes the value... since most browsers will support Javascript.
Don't know of anything offhand but i'm going to have a dig and see if i can find a lightweight JS solution.
As said though, if you're already loading jQuery (or a plugin already does) then you can proberly whip it up in a few lines....
You'll also find various themes do this, download one, open the file that handles the search box and see how they do it... (that's the easy way)..