I find it most useful to define referential transparency in terms of substitution (which is the definition Wikipedia starts with). Namely: if I substitute a function call with its result is there an observable different in program behaviour? If there is, the program is not referentially transparent.

This is clearly the case with `println`.

   (println "Hi there")

is clearly not the same program as

   nil

as the former causes output to the console, while the latter does not -- an observable difference.

Using Scheme (because I don't know Clojure that well)

   (define (foo x)
     (let ([y 0])
       (set! y 10)
       (+ x 1)))

is a referentially transparent program. Although it contains a side-effect (the assignment to y) this effect is not observable outside `foo`, and thus any call to `foo` can be substituted with its result.