Clear text in specific XML node in Scala?

As in your previous question, you can use a RewriteRule in conjunction with RuleTransformer for this. Given the particular form of your tags, you can directly pattern match on them.

val removeText = new RewriteRule {
  override def transform(n: Node): NodeSeq = n match  {
    case <a:updated>{_}</a:updated> => <a:updated></a:updated>
    case _ => n
  }
}

val output = new RuleTransformer(removeText).transform(xml)

However, note that your input XML is a bit malformed - the <a:category> tags are never closed. That corrected, I got the following interaction in scala:

scala> val xml= <a:feed>
     |              <a:id>http://localhost:8089/</a:id>
     |              <a:entry>
     |                  <a:id>test1</a:id>
     |                  <a:title/>
     |                  <a:summary/>
     |                  <a:updated>2016-07-05T13:34:37Z</a:updated>
     |                  <a:author>
     |                  <a:name/>
     |                  </a:author>
     |                  <a:link rel="edit" href="test1"/>
     |                  <a:category/>
     |              </a:entry>
     |              <a:entry>
     |                  <a:id>test2</a:id>
     |                  <a:title/>
     |                  <a:summary/>
     |                  <a:updated>2016-07-05T13:34:39Z</a:updated>
     |                  <a:author>
     |                  <a:name/>
     |                  </a:author>
     |                  <a:link rel="edit" href="test2"/>
     |                  <a:category/>
     |              </a:entry>
     |          </a:feed>
xml: scala.xml.Elem = ..

scala> val output = new RuleTransformer(removeText).transform(xml)
output: Seq[scala.xml.Node] =
         <a:feed>
             <a:id>http://localhost:8089/</a:id>
             <a:entry>
                 <a:id>test1</a:id>
                 <a:title/>
                 <a:summary/>
                 <a:updated></a:updated>
                 <a:author>
                 <a:name/>
                 </a:author>
                 <a:link rel="edit" href="test1"/>
                 <a:category/>
             </a:entry>
             <a:entry>
                 <a:id>test2</a:id>
                 <a:title/>
                 <a:summary/>
                 <a:updated></a:updated>
                 <a:author>
                 <a:name/>
                 </a:author>
                 <a:link rel="edit" href="test2"/>
                 <a:category/>
             </a:entry>
         </a:feed/>