As a previous answer says, if you're working on Linux, you can't retrieve them. They're gone: that's the point of "git clean". Unless you're using a filesystem capable on doing snapshots but then, you would probably know how to get them back.
If your goal was to avoid seeing them in "git status", you have two possibilities:
A)
git status -uno
does not print untracked files and directories.
B) If there are a fixed set of files (or a filename pattern) that you will never want to see in "git status" nor put them under versioning, you can "ignore" them. You simply have to put their name or pattern in a file named ".gitignore" at your repository's root.
This is typically used for generated files.
The following example ignores filenames ending with ".html" and the file called "out/myFile.txt":
*.html
out/myFile.txt
See
git help ignore
for more info.