$('tr.load_ua').click(function() {
      $.get('<?php echo site_url(); ?>/espace_client/content_line/?id_ua='+$(this).attr('id'),
        function(data){
        //Data OK
          //alert("Data Loaded: " + data);

        // not ok, why he doesn't do anything ? why not loading the ajax content after the current element ?
          $(this).after(data);
        }
      );


// ajax call return OK
<tr><td colspan="3">test</td></tr>