Would a lobster kill a clown

No, they are an opportunistic predatory. If the chance arises, he will take it. But clowns are pretty active fish, he would probably be fine. However any basking fish (like gobies or blennies) would have a much higher chance of being eaten.
Im thinking of putting a lobster in my refugium all by his lonesome, and the refugium will be on its night cycle while the real world and the display tank are on day cycle so I may catch a glimpse of him here and there :)

but like little_fish said, he very well may kill pretty much anything he can get his claws on, so he is not so fish friendly
I have one. I have seen his whole body twice. Once when i put him in and once randomly one day. Pretty, but its a bummer only seeing an animals claws for a nano second.
Like my pistol shrimp..I only see his antenna LOL But if I look under my tank, I do see him crawling through his tunnels ;)
As others have said, a Lobster if given the chance, and was hungry enough would possibly do so. Though, because Clownfish are active fish, and seemingly swim at levels in contrast with Lobsters, I think it would be a fine combination. However, it's definitely something you would have to keep an eye on.
I've got a lobster in my tank with my clown and damsel....no problems at all. The fish swim all around his little hideout all the time and hes never shown any aggression towards them. He pretty much just hides all the time.