Just because I apparently can't think in an interview room.

longestsubstring.clj

(defn findSubstring
  "Finds the longest substring"
  [s]
  (->> (range (dec (count s)) 0 -1)
       (map (fn [i]
              (->> s
                   (partition i 1)
                   (map clojure.string/join)
                   getSubstring)))
       (drop-while nil?)
       first))

(defn getSubstring
  "returns the string if substring match otherwise nil"
  [candidates]
  (->> candidates
       sort
       (partition 2 1)
       (filter (partial apply =))
       ffirst))