I got it working with .-currentTarget
. See http://ckirkendall.github.io/enfocus-site/#doc-events and http://docs.closure-library.googlecode.com/git/class_goog_events_Event.html.
(defaction change-src [selector src]
[selector] (ef/set-attr :src src))
(defaction thumb-hover []
[".thumb"] (events/listen :mouseenter
#(change-src "#full-image"
(ef/from (.-currentTarget %)
(ef/get-attr :src)))))