You are correct about CLIP. It's invisible and solid to players only -- not bullets etc. You don't need an entity to use CLIP, just put the texture on a brush.

You should not use HINT and SKIP unless you have a very good understanding of how the compilers (hlbsp and hlvis) work. Basically you can use them to force hlbsp to cut an "area" into smaller chunks. If done correctly this can increase the speed and efficiency of your map, but usually it will just hurt.

I had to use a HINT and SKIP extensively in de_salt2 to even get it to compile because the map has odd angles everywhere that confuse hlbsp. This is an extremely rare use of it though.

If you want to try to understand HINT (and SKIP, since they're always used together) read this:
http://twhl.co.za/tutorial.php?id=71