To do this you will need to use a scripting language, probably javascript. Here an example

<input type='text' value'Some text' onclick='javascript: this.value = ' />

Hope this helps.

Edit:

To meet what David is explain here is a second example in case that is what you are looking for

<script type='javascript'>

    var clear = true;

    function clear(obj)

    {

        if(clear)

        {

            obj.value = '';

            clear = false;

        }

    }

</script>



<input type='text' value'Some text' onfocus='clear(this);' />