Add Define Something Only Once as a clojure til

clojure/define-something-only-once.md

@@ -0,0 +1,22 @@
+# Define Something Only Once
+
+Clojure provides [`defonce`](https://clojuredocs.org/clojure.core/defonce)
+which is a macro that defines something only once. Once a variable has been
+bound to a value, subsequent attempts to do `defounce` for that variable
+will go unevaluated. This will have no implications for how the `def`
+special form works.
+
+Here is an example:
+
+```clojure
+(defonce stuff 5)
+#'user/stuff
+user=> (defonce stuff "what")
+nil
+user=> stuff
+5
+user=> (def stuff "okay")
+#'user/stuff
+user=> stuff
+"okay"
+```