The clocks are mainly meant as output fields, but in some situations they also accept mouse clicks. What the clicks do varies. They can be taken as a signal you want to claim the game because the opponent has flagged (when his clock displays a negative number). When you keep the Shift key pressed during the click, you can adjust the clocks by adding (right-click) or subtracting (left-click) a minute.
Testing with xboard 4.7.1 ,here it seems both left and right mouseclicks will decrement the clock.
I do something wrong or has something been changed?
Best
Fonzy