subset.clj

(defn pairs-seq [m]
  (->> (keys m)
       (mapcat #(partition 2 (interleave (repeat %) (get m %))))))

(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) ;=> {:y [3 4], :z [5]}