Activate 'lisp' mode with:
M-x lisp-mode

To get an interactive session with Emacs LISP, enter emacs and run:

M-x ielm

To run clisp within emacs, add /opt/clisp/bin to your path and add the
following line to your ~/.emacs file:

(setq inferior-lisp-program "clisp")

...and restart emacs. The documentation at this page:
http://www.gnu.org/manual/emacs-20.3/html_node/emacs_314.html
will then apply; if you just want to get into clisp within emacs, M-x
run-lisp will do the trick.