Remove as much standing water and water in the drain opening as possible. Bring a large container of water to a hard boil and pout into the drain. Don't pour it all at once just a little to get it started then a lot more. Once it is draining then run tap water as hot as possible for at least 3 minutes. Doing a load of laundry right after this would also be helpful to flush the main line drain of the grease.
If the initial boiling water doesn't release the grease you will need to purchase a commercial drain cleaner product from the local home improvement store.
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