(defn compare-ab [x y]
  (compare [(get x :a) (get x :b)]
           [(get y :a) (get y :b)]))

(def ss-ab (apply sorted-set-by compare-ab
                      [{:a 42 :b 5000}
                       {:a 1 :b 2}
                       {:a 99 :b -1000}
                       {:a -1 :b 7}]))
user=> ss-ab
#{{:a -1, :b 7} {:a 1, :b 2} {:a 42, :b 5000} {:a 99, :b -1000}}

user=> (subseq ss-ab compare-ab 5)
({:a -1, :b 7} {:a 1, :b 2} {:a 42, :b 5000} {:a 99, :b -1000})

user=> (subseq ss-ab > 5)
({:a -1, :b 7} {:a 1, :b 2} {:a 42, :b 5000} {:a 99, :b -1000})

It's not selecting a subsequence....