How to create default value for function argument in Clojure

A function can have multiple signatures if the signatures differ in arity. You can use that to supply default values.

(defn string->integer 
  ([s] (string->integer s 10))
  ([s base] (Integer/parseInt s base)))

Note that assuming false and nil are both considered non-values, (if (nil? base) 10 base) could be shortened to (if base base 10), or further to (or base 10).