Recently, someone in this mailing-list pointed to the problem that CLX was not loaded during McCLIM compilation process in CLISP. Indeed, :clim-clx loading for was disabled for CLISP. I fix it.
CLISP has it's own two implementations of CLX -- new-clx and mit-clx. Their presence can be enabled or disabled with CLISP configure script's options --with-module=clx/new-clx or --with-module=clx/mit-clx. First, we verify availability of :xlib package. If :xlib doesn't exist, then we load CLX from ASDF. Otherwise, some heuristic applied to detect what implementation provided with CLISP: if the symbol `set-selection-owner' is not declared as exported, we have implementation (new-clx, I suppose) that currently cannot be used as McCLIM backend. In this case, we unlock :xlib and try to replace it with CLX from ASDF.
I tried mit-clx (predecessor of telent-clx) as McCLIM's backend two years ago, and it worked. But, AFAIR, I read (I don't remember where) that something (and I don't remember what) would not work with this implementation and also that the implementation hasn't being updated for a long time. I think the right decision is to leave it as possible backend for a while and don't replace it with telent CLX from ASDF.