1. Using twitter-names as a global constant is fine. It's like a global constant function. When you need to update the map, add new attributes for the same key, etc. then revisit the issue. Until it bothers your code, don't sweat it.

2. Using 'first' – it is not uncommon in Lisp code to do something line (def get-foo first) where the name get-foo communicates the intent better for your application. Then too if the implementation of get-foo changes you don't have to go through your code to determine whether each use of 'first' should change. In that case just redefine get-foo.

3. Formatting defn's – if you can put a defn on a single line without impacting readability, do so. Don't impinge on readability or editability.

filter and map are not always the handiest tools. Especially when your tree is not nicely flat and regular like this feed.

That's why Chouser authored zip-filters (xpath-like selection) and I created Enlive (css-like selection and transformation).

sample code to retrieve all items of a feed:
(-> your-url duck-streams/reader (enlive/select [:item]))

so something like this:
"(def authors (map #(get-author %) rss-titles))"

can instead be written like this:
"(def authors (map get-author rss-titles))"