Brifter loose even with bolt tight
First thing to do is loosen the bolt all the way and grease the threads. If possible, snake a lube dripper in and get a drop of lube under the bolt shoulder too. That may completely fix it, but while you have the lever separate from the clamp, look everywhere you can see for cracks in the lever body that might be preventing the clamp from snugging everything up properly. Pull back the hood as needed to inspect. If none of that leads to an answer, next thing to do would be get better eyes on the section of bar it's trying to clamp, and also the clamp itself.

Given this started with a crash I'd be pretty worried that the lever body is broken somewhere.