I think the below code should work
<%: Html.ActionLink("item.headerclob, "ViewTopic","Forum" ,
new { id=item.topicId },null) %>
it use the following format
public static string ActionLink(this HtmlHelper htmlHelper,
string linkText,
string actionName,
string controllerName,
object values,
object htmlAttributes)
if you are using MVC 3 then the you can just use "item.topicId" instead of "id = item.topicId"
Edited
yes it works but after removing semicolon from item.headerClob
<%: Html.ActionLink(item.headerclob, "ViewTopic","Forum" ,
new { id=item.topicId },null) %>
Edit
add a class to action link then use your css file for setting necessary properties
<%: Html.ActionLink(item.headerclob, "ViewTopic","Forum" ,
new { id=item.topicId , @class = "YourClass"},null) %>
now you can apply css properties to the action link you set css properties to others
EDIT
If you do not want to use razor, I could suggest you to build anchors by your self like following
<a href="<%=Url.Action("ViewTopic", "Forum",new { id=item.topicId})%>" class="YourClass"> item.headerclob </a>