Your JavaScript:
function clearContents(element) {
element.value="";
}
And your HTML:
<textarea onfocus="clearContents(this);">Please describe why</textarea>
I assume you’ll want to make this a little more robust, so as to not wipe user input when focusing a second time. Here are five related discussions & articles.
And here’s the (much better) idea that David Dorward refers to in comments above:
<label for="explanation">Please describe why</label>
<textarea name="explanation" id="explanation"></textarea>