If you do want to use the primary selection, you can use `xclip -l 1 -quiet`, which will remain in the foreground until a single request is made for its contents (a single paste); it will then quit, and the content will no longer be available.

Not fool-proof, but helps mitigate the risk. If an attacker snoops on the selection, it'll quit before your paste and you'll know that something is wrong.