0:28:36 in "Start Clojure" by Chas Emerick on Youtube:
You have to start app in texteditor of app.clj instead of the project "startclojure". Otherwise you have to use
(def server (ring.adapter.jetty/jetty/run-jetty #'startclojure.app/app {:port 8080 :join? false}))
instead of
(def server (jetty/run-jetty #'app {:port 8080 :join? false}))