org-task-overrun-text

org-task-overrun-text is a variable defined in `org-clock.el'.
Its value is

nil

• This variable is an alias for `org-clock-task-overrun-text'.

Documentation:
Extra mode line text to indicate that the clock is overrun.
The can be nil to indicate that instead of adding text, the clock time
should get a different face (`org-mode-line-clock-overrun').
When this is a string, it is prepended to the clock string as an indication,
also using the face `org-mode-line-clock-overrun'.

You can customize this variable.