It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import at the very top of the file.

ZenScript
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import crafttweaker.api.item.type.tiered.TieredItem;

Extending ItemDefinition

Link to extending-itemdefinition

TieredItem extends ItemDefinition. That means all methods available in ItemDefinition are also available in TieredItem

Name: getTier

Gets the tier of this item.

Returns: The IItemTier of this item.
Return Type: Tier

ZenScript
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// TieredItem.getTier() as Tier

myTieredItem.getTier();