The listen keycommand only works when a mixer has focus. Here is how you can instantly toggle Listen on a selected track in the arrange while never having to bring up the mixer (this assumes one uses the control room):
set up a two command macro that first toggles the control room, and then toggles listen, and assign a kc to it. By toggling control room, the listen command at once becomes available, without the need to toggle the actual mixconsole, which the less you see of, indeed the better (also it is considerably faster).