This is a long-running issue in Drupal, with no perfect solution. Using <front> may work for you, but I doubt it. There are some modules that address this (I'll leave the search to you). What I did in the past was to put a small text on a page that says "Here's what you can find in this section" and then include a menu block for the sub-menu of the main item.

Nancy

From: "Philip_Wetzel@nhd.uscourts.gov"
My client wants to have no landing page if the user clicks on any of the
top level menu links. Specifying <front> for the path works perfectly, except the color of the
menu becomes the "selected" color (same as mouse over).