You'll need to reset the value of the field, then trigger the liszt:updated event on the input to get it to update, ive made a fiddle with a working example here.

http://jsfiddle.net/VSpa3/3/

$(.chzn-select).chosen();

$('a').click(function(){

    $(.chzn-select).val('').trigger(liszt:updated);

});​

Since the release of chosen v1.0 the trigger is now called 'chosen:updated'. Anyone using this new version needs to trigger the update using

$(.chosen-select).val('').trigger(chosen:updated);