gtk:text-buffer-bounds (buffer)

buffer -- a gtk:text-buffer object
start -- a gtk:text-iter iterator for the first position in the text buffer
end -- a gtk:text-iter iterator for the end position in the text buffer

Retrieves the first and last iterators in the text buffer. The entire text buffer lies within the range [start, end). Get the start and end itererator for a text buffer and clear the text buffer.

(defun clear-buffer (buffer)
  (multiple-value-bind (start end)
      (gtk:text-buffer-bounds buffer)
    (gtk:text-buffer-delete buffer start end)))