Noticed the problem with cl-slime-swank, but seems to happen with a variety of packages including sbcl-cl-uglify-js itself. I checked javascript.scm, which does import lisp-xyz; Haven't done any deeper debugging to discover possible causes.
Backtrace follows. $ guix package --show=sbcl-cl-uglify-js Backtrace: In ice-9/boot-9.scm: 222:29 19 (map1 (((gnu packages gettext)) ((gnu packages #)) (#) …)) 222:29 18 (map1 (((gnu packages ghostscript)) ((gnu packages …)) …)) 222:29 17 (map1 (((gnu packages gl)) ((gnu packages glib)) ((…)) …)) 222:29 16 (map1 (((gnu packages glib)) ((gnu packages gnome)) # …)) 222:29 15 (map1 (((gnu packages gnome)) ((gnu packages gnupg)) # …)) 222:29 14 (map1 (((gnu packages gnupg)) ((gnu packages #)) ((…)) …)) 222:29 13 (map1 (((gnu packages gstreamer)) ((gnu packages #)) # …)) 222:29 12 (map1 (((gnu packages gtk)) ((gnu packages image)) (#) …)) 222:29 11 (map1 (((gnu packages image)) ((gnu packages #)) ((…)) …)) 222:29 10 (map1 (((gnu packages imagemagick)) ((gnu packages …)) …)) 222:17 9 (map1 (((gnu packages javascript)) ((gnu packages #)) …)) 2800:17 8 (resolve-interface (gnu packages javascript) #:select _ …) In ice-9/threads.scm: 390:8 7 (_ _) In ice-9/boot-9.scm: 2726:13 6 (_) In ice-9/threads.scm: 390:8 5 (_ _) In ice-9/boot-9.scm: 2994:20 4 (_) 2312:4 3 (save-module-excursion #<procedure 7fbf1f28b330 at ice-…>) 3014:26 2 (_) In unknown file: 1 (primitive-load-path "gnu/packages/javascript" #<proced…>) In gnu/packages/javascript.scm: 460:10 0 (_) gnu/packages/javascript.scm:460:10: error: sbcl-cl-uglify-js: unbound variable