In the first part of the movie when all seven kids are in the sewer with Penny-wise, Beverly shoots him with a piece of silver and a slingshot, causing him to go down the drain. After this you can clearly see Bill jump forward and grab his hand.
I was thinking the same thing. Yet, it does make since that Bill did this to try to destroy him even more because in the book he wanted to go after it because he did not think he was dead. As he says in the movie.
I have seen the movie many times and never did understand why Bill would want to grab the clown. Perhaps the clown was supposed to grab him, but it looks like it happened the other way around.