There's a missing leading space in the mode line clock display when the current task does not have an effort value set.

In org-clock.el, `org-clock-get-clock-string'.

Emacs  : GNU Emacs 25.1.1 (x86_64-pc-linux-gnu, GTK+ Version 3.10.8), modified by Debian

Package: Org mode version 9.0 (9.0-elpa @ ~/.emacs.d/elpa/org-20161102/)