org-clock-heading-function

org-clock-heading-function is a variable defined in `org-clock.el'.
Its value is

nil

• This variable may be risky if used as a file-local variable.

Documentation:
When non-nil, should be a function to create `org-clock-heading'.
This is the string shown in the mode line when a clock is running.
The function is called with point at the beginning of the headline.

You can customize this variable.