The text formatting cleanup functionality is provided through the Remove Format plugin which by default is available in the Standard and Full distributions.
When the plugin is enabled, thebutton is automatically added to the toolbar. You can use it to remove any text formatting that is applied through inline HTML elements and CSS styles, like basic text styles (bold, italic, etc.), font family and size, text and background colors or styles applied through the Styles drop-down. Note that it does not change text formats applied at block level.
Since this feature applies to inline styles, you need to select the text fragment whose formatting is to be removed.