gistfile1.clj

(def five-by-five [[:. :. :. :. :.]
                   [:. :. :. :. :.]
                   [:. :. :. :. :.]
                   [:. :. :. :. :.]
                   [:. :. :. :. :.]])

(defn to-coordinates [board]
  (into []
   (flatten
    (map-indexed
     (fn [row-pos row]
       (map-indexed
        (fn [col-pos piece]
          {:row-pos row-pos
           :col-pos col-pos
           :piece piece}) row)) board))))

Something like this?

(defn to-coordinates2 [board]
  (mapcat (fn [col col-idx]
            (map-indexed #(array-map :col-pos col-idx :row-pos %1 :piece %2) col))
          board
          (range)))

Or, equivalently:

(defn to-coordinates3 [board]
  (mapcat (fn [col col-idx]
            (map-indexed (fn [row-idx piece]
                           {:col-pos col-idx :row-pos row-idx :piece piece})
                         col))
          five-by-five
          (range)))