subset.clj

(defn interleave*
  "Like interleave, but doesn't quit on the shortest seq."
  [& colls]
  (when-let [have-stuff (seq (keep seq colls))]
    (lazy-seq (concat (map first have-stuff)
                      (apply interleave* (map rest have-stuff))))))

(defn pairs-seq [m]
  (->> (keys m)
       sort
       (map #(partition 2 (interleave (repeat %) (get m %))))
       (apply interleave*)))

(defn subset [m n]
  (reduce #(update-in %1 [(first %2)] (fnil conj []) (second %2)) {}
          (take n (pairs-seq m))))

(def m1 {:x [1 2] :y [3 4] :z [5]})

(subset m1 3) ;=> {:x [1], :y [3], :z [5]}