First setup

module/sample-code.js

@@ -0,0 +1,24 @@
+ html.find('.item .item-name h4').click(event => this._onItemSummary(event));
+ 
+  /**
+   * Handle toggling of an item from the Actor sheet
+   * @private
+   */
+  _onItemSummary(event) {
+    event.preventDefault();
+    let li = $(event.currentTarget).parents(".item"),
+        item = this.actor.getOwnedItem(li.data("item-id"));
+
+    // Toggle summary
+    if (item.data.data.description !== undefined && item.data.data.description !== null){
+      if ( li.hasClass("expanded") ) {
+        let summary = li.children(".item-summary");
+        summary.slideUp(200, () => summary.remove());
+      } else {
+        let div = $(`<div class="item-summary">${item.data.data.description}</div>`);
+        li.append(div.hide());
+        div.slideDown(200);
+      }
+      li.toggleClass("expanded");
+    }
+  }