You can use the Aromatic Honey Brew right as you walk into the Dalaran well, and you will walk down the well, through some walls and if you're lucky end up in the room with the Underbelly Croc.
[Edit] - You can use the brew next to any of the gates in the Underbelly to get through them. It helps if you are a mage and can ice block once you're through, so you don't go back out.