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Depending on how much of it broke off will determine if it's repairable. If there is enough of a nub left to attach some airline tubing to it securely, you can use super glue to attach it permanently and then add a piece of rigid tubing to act as the new outlet. If there is no nub left, you may need to contact Tetra directly to see if they even offer a replacement piece for your pump.
( If you do end up using glue, make sure you run the air pump for a couple of hours with the new outlet open so that any fumes that may come from the glue are released into the air and not into the aquarium. )