Use the cheshire library found here and use the generate-string function with the pretty flag set to true
Example
;; generate some JSON with pretty formatting
(generate-string {:foo "bar" :baz {:eggplant [1 2 3]}} {:pretty true})
;; {
;; "foo" : "bar",
;; "baz" : {
;; "eggplant" : [ 1, 2, 3 ]
;; }
;; }