(let [xx (vec nil)]
 (doseq [kv @clients]
   (if (= "first" (:room-id (val kv)))
     (conj xx (:user-id (val kv)))))
  (println xx))

;the better would be instead of var xx, return only data like:
(into [] (:user-id (val kv)))
;but both returns nil :/