Is it possible to define a default text for description of workitem type Defect?

Instead of using the script-base default value, you can also create multi-line text default values and map them to each work item type. See https://jazz.net/wiki/bin/view/Main/AttributeCustomization#Default_values. This requires more default values to be created and maintained, but it's another option to consider.

I also attach a sample script below. The script assumes the ID of defect work item type to be 'defect'. The script is based on the skeleton provided in the 'Script-based default value' section in the jazz.net wiki link that Dinesh posted above. To create a script-based default value and use it, see https://jazz.net/wiki/bin/view/Main/AttributeCustomization#Using_scripts_for_attribute_cust

//========== SAMPLE SCRIPT-BASE DEFAULT VALUE ==========
dojo.provide("com.example.common.MyClass");
dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes");

(function() {
var WorkItemAttributes= com.ibm.team.workitem.api.common.WorkItemAttributes;
   
    dojo.declare("com.example.common.MyClass", null, {
    getDefaultValue: function(attribute, workItem, configuration) {
     
        var WorkItemType= workItem.getValue(WorkItemAttributes.TYPE);
        var DEFECT_DESC= "Defect Description Line 1\n\
Defect Description Line 2"
        var OTHER_DESC= "Other Description Line 1\n\
Other Description Line 2"

            if (WorkItemType == "defect") {
                return DEFECT_DESC;
            }
           
            return OTHER_DESC; 
    }  
});
})();

have you explore the "Script Based Default Value" which seems more appropriate to me...

in the script, you could check for the workitem's TYPE (workItem.getValue(WorkItemAttributes.TYPE);) and return a different value accordingly. 

more on script based default could be found here : https://jazz.net/wiki/bin/view/Main/AttributeCustomization#Script_based_default_values

hope it helps.