clear-emacs-whitspace.el

(defun delete-trailing-whitespace-except-current-line ()
  (interactive)
  (let ((begin (line-beginning-position))
        (end (line-end-position)))
    (save-excursion
      (when (< (point-min) begin)
        (save-restriction
          (narrow-to-region (point-min) (1- begin))
          (delete-trailing-whitespace)))
      (when (> (point-max) end)
        (save-restriction
          (narrow-to-region (1+ end) (point-max))
          (delete-trailing-whitespace))))))

(add-hook 'before-save-hook 'delete-trailing-whitespace-except-current-line)