@xsc's comment pointed to a possible duplicate, still I'll follow his advice on posting an answer myself to keep a reference for Clojure specifics.
Here's what I ended up with:
(def baos (java.io.ByteArrayOutputStream.))
(-> (SomeClass. baos) .export)
(String. (.toByteArray baos) (. java.nio.charset.Charset defaultCharset))
See the comments in this answer for more about Charset.