Here are some smaller clips of scary movies (get those cushions ready!):

An American Werewolf in London. the tube scene:[link]

Alien. dallas in the duct scene. I rate Alien as a horror first and foremost and this is one of the scarier scenes.
Alas I read some of the comments below this clip and 'Death by jazz hands' was one of them. I couldn't stop
laughing: [link]

the end of John Carpenter's Halloween. Laurie hides in a tragically flimsy closet in order to escape an inhumanly strong psycho villain: [link]

The blood test scene in john Carpenter's The Thing (warning: quite gruesome this one) : [link]

Are clowns scary. Yes, yes they are. Here's a scary clown compilation which has IT, poltergeist and Killer Klowns from Outer Space (my God,
that's a blast from the past!):[link]