(with-open-file (str "/.../filename.txt"
                     :direction :output
                     :if-exists :supersede
                     :if-does-not-exist :create)
  (format str "write anything ~%"))

You may also choose different settings for the with-open-file macro. If you use :append instead of :supersede then you can write into the text file while preserving its context instead of superseding the available content.