This probably does what you want:

  bindkey '^[' complete-word

You can also use "expand-or-complete", which is what Tab is mapped to by default and will also expand things (e.g. "ls *<Esc>" will expand the "*").

You may want to set lower KEYTIMEOUT too, so it doesn't wait as long for further keys after an escape. I'm not sure what the default is.