Hi Andrea,

I tried JavaScript and it does not work. This script should do it:

/*******************************************************************************
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 * (c) Copyright IBM Corporation 2011. All Rights Reserved.
 *
 * Note to U.S. Government Users Restricted Rights:  
 * Use, duplication or disclosure restricted by GSA ADP Schedule 
 * Contract with IBM Corp. 
 *******************************************************************************/
dojo.provide("com.example.CleanListValueProvider");
dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes");

(function() {
	var WorkItemAttributes = com.ibm.team.workitem.api.common.WorkItemAttributes;
    dojo.declare("com.example.CleanListValueProvider", null, {

        getValue: function(attribute, workItem, configuration) {
            //var category = workItem.getValue(WorkItemAttributes.FILED_AGAINST);
            var category = workItem.getLabel(WorkItemAttributes.FILED_AGAINST);
            console.log("Category: " + category);
            if (category=="Core"){
            	 //var result= [];	
            	 var result= null;	
			     return result;
			}
			// var result= null; // Works but not desired
			return workItem.getValue(attribute); // java.lang.String incompatible with java.util.Collection
        }
    });
})();

Unfortunately it does not.  I can basically not even return the original value of the attribute (to not change the value in case it is not supposed to be). I think List attributes are currently at least not supported in JavaScript based calculated values. They work only for value sets. Feel free to file a defect or enhancement request.

I can only return null to clear the values, but since I always have to return a value, that means it will always clear the attribute if the category changes. Not very helpful.

you could try to follow https://rsjazz.wordpress.com/2013/06/26/attribute-customization-java-based-value-providers-conditions-and-validators/ and create a calculated value in Java. In this case you can return a List or Collection of string values, which should work.