Clojure MapReduce Reducer
probably a bit too late to help OP, but in case anyone else stumbles upon this question, here's a nice succinct way of doing it, using the frequencies function:

(doseq [[word freq] (frequencies
                      (map
                        #(re-find #"^[^\t]+" %) ;; just get the first non-tab characters
                        (line-seq (java.io.BufferedReader. *in*))))]
  (println (str word "\t" freq)))