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A thin smear of pretty much any 2-part epoxy should do this fine. You might want to dry the teapot out in the oven first - a few hourst at 120oC or so should help.
If you're keen, you could build the damaged part back up - if your teapot is black (like any self respecting teapot) then JB-weld is a black metal-loaded epoxy that might do what you want. Build a dam around where you want it out of sellotape. Post a pic if you want actual useful advice.
Me, I'd just leave it, as long as it still pours OK. Unless you're particularly sickly, nothing bad's going to live on a bit of pottery, especially if it's hosed down with tea.