If it's just one instance in the code, drop the span part, give the <a> an id and add 2 lines of code to the css file.
<a href="http://www.myurl.com" id="thisone">CLICK HERE</a>
a#thisone:link, a#thisone:visited { font: 700 12px Arial, sans-serif; color: #fff; }
a#thisone:hover, a#thisone:active { color: #000; }