At one point Red Hat paid to get one of their releases "officially" tested as conforming to the POSIX standard, and passed, and was thus able to officially call that version of Linux "Unix(TM)". I may be mistaken, but I haven't heard them pushing that any time recently, and the market doesn't seem to care, so I doubt they've continued jumping through that hoop.

Now, if you want to take a deeper interpretation of the analogy of "clone", as in duplicate DNA, then Linux is less of a clone of "Unix" than BSD, since it was independently written, and a judge has pretty much declared that it contains no copyrighted content that originated with "Unix". Read the SCO travesty timeline at groklaw.com. Meanwhile, the Berkeley (Berkeley Standard Distribution - BSD) folks actually worked from a copy of the original Bell Labs Unix - but worked so hard on it that in a much earlier court case, a judge made a (sealed) declaration that BSD had only a handful of lines left from the original code, and that Berkeley could distribute their work, with the exception of that handful, on their own terms. Berkeley quickly replaced/removed any infringing content, and that became the basis for the *BSDs.

For the glorious history of Unix, read Peter Salus's "A Quarter Century of Unix".