The lisp-y answer is, who cares if there isn't syntax, just add your own.
(defmacro awhen (test &body body)
`(let ((it ,test))
(when it ,@body)))
and then use it
(awhen (expensive-computation)
(format t "~a~%" it))
This class of macros are often prefixed with an "a" for anaphoric. See aif
for Paul Graham's examples in On Lisp.