"Clean install" and "without losing your data" are two mutually exclusive things.
A clean install means erasing everything and installing a brand new OS, with no user data, settings, third-party software, etc.
"Not losing your data" means keeping everything (except the new OS) as it was.
Since Snow Leopard, the OS installs "in place" -- what used to be the option of "Archive & Install" -- by default. In other words, it just updates the OS and keeps all your files, settings and software intact.
But of course you will have a backup anyway, in case something goes wrong. Won't you?
However, you can't do a normal install to DOWNGRADE the OS. First of all, you need to check that your machine can run Snow Leopard. If it is too new, then SL won't have the necessary drivers inside it.
If you are downgrading, then you would be better off doing a clean install and then migrating your user data from your backup.