A Lisp runs Lisp code, code that could have been written anywhere from 50 years ago to today. The extent and degree of modifications required before that code runs, illustrates how far away one is from Lisp.

All Lisps (incl Scheme, Common Lisp, Emacs Lisp) sit fairly close together in this historical "Lisp" space. I can get Lisp code from the CMU AI repository and run it as is or with slight modifications in any of the aforementioned Lisps, but Clojure would require a complete rewrite, both syntax and logic/semantics. Ergo, I and lots of others don't see Clojure as a Lisp. I'd call it Lisp-inspired or a descendant of Lisp, but it's clear that it is a different language.