Clear Channel Radio Ad Revenue Slips  NEW YORK (Reuters) - Clear Channel Communications Inc.  &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=CCU.N target=/stocks/quickinfo/fullquote"&gt;CCU.N&lt;/A&gt;, the largest U.S. radio station chain, on Friday posted  higher operating earnings although radio advertising revenue  slipped on weak sales to its automotive and telecommunications  customers.