I have a file with cloned objects and when I print it, or export it to PDF, the clones does not show up. Is it a bug or a feature?

Do the originals of the clones have filter effects applied or are they blurred? If so, yes, it's a bug (fixed in 0.48): Bug #376987 in Inkscape: “Clones from blur elements are not visible in exported PDF”.

If not - I don't know.

Yes, it is a clone with blurred effect. Thanks for the information.