Find ocurrences of items

find_items_ocurrence.clj

(let [my_list '(a b b c a a c) my_set (set my_list)]
  (zipmap
    my_set    
    (map
       (fn [y] 
          (count
            (filter 
              (fn [x] (= x y)) my_list
            ) 
          )   
       ) my_set
    )
  )
)

(def l '(a b b c a a c))
(defn inc* [x] (if-not x 1 (inc x)))
(reduce (fn [v v2] (update-in v [v2] inc*)) {} l)

Una versión más idiomática de tu solución

(let [my-list '(a b b c a a c)
      ks (set my-list)]
  (into {} (map (fn [y] [y (count (filter #{y} my-list))]) ks)))

;; O también

(let [my-list '(a b b c a a c)
      ks (set my-list)]
  (zipmap ks (map (fn [y] (count (filter #{y} my-list))) ks)))

;; Hecha función

(def my-list '(a b b c a a c))

(defn count-ocurrences [my-list]
  (let [ks (set my-list)]
    (into {} (map (fn [y] [y (count (filter #{y} my-list))]) ks))))

(clojure.pprint/pprint (count-ocurrences my-list))

(defn freq [the-list] (reduce #(assoc %1 %2 (inc (%1 %2 0))) {} the-list))

Similar a la mía pero igual válida 😄
Que viva la programación funcional 🎉