Idiomatic letterpress cheater
Well there is a standard library function: frequences :)

user&gt; (doc frequencies)
-------------------------
clojure.core/frequencies
([coll])
  Returns a map from distinct items in coll to the number of times
  they appear.


Since in Clojure many many types can be converted to sequences, frequencies will work on many types, including strings. In Clojure you can get an amazing amount of work done simply by using the sequence library. So your string-to-letter-hash is actually just an example of frequences.

For your can-play: the same type of thinking applies. Try to resist your first impulse of writing a loop. The Clojure library functions will almost always do the job - the thing that requires training is knowing the library well enough to find a good way to compose the functions.

(defn can-play? [word freqs]
  (every? #(&gt;= % 0) (vals (merge-with - freqs (frequencies word)))))


(updated to comment version: https://gist.github.com/4105644)