Georgie is convinced by the clown to reach down into the sewer to get his boat back, at which point the clown strikes, ripping off the boy’s arm.
If you’ve read the book It or seen the original TV miniseries then you’ll know the scene in question.
In it, two brothers, Bill and Georgie, build a paper boat that Georgie can float down the gutters filled with water due to a storm.
In the book, Georgie’s boat is washed into the sewer and upon trying to retrieve it, the young boy is confronted by Pennywise the clown for the first time, peering up at him from below the sewer grate.