Solution to problem - https://www.youtube.com/watch?v=6NvNtYChIo8

replace-digits.clj

(defn shift
  "Takes digit char, returns int offset from zero."
  [nbr]
  (- (int nbr) (int \0)))

(defn next-pair
  "Reducing fn, takes acc, the destructured pair [chr nbr]
  and returns next acc."
  [acc [chr nbr]]
  (conj acc chr (if nbr (char (+ (int chr) (shift nbr))))))

(defn replace-digits
  "Takes alpha-digit string, returns digit replaced string."
  [s]
  (apply str (reduce next-pair [] (partition-all 2 s))))

;; For those who like brevity
(defn replace-digits [s]
  (apply str
         (reduce (fn [acc [chr nbr]]
                     (conj acc chr
                           (if nbr
                               (char (+ (int chr)
                                        (- (int nbr)
                                           (int \0)))))))
                 []
                 (partition-all 2 s))))