Staying late at the office to discuss shorter working hours rather defeats the object of the exercise.
You can substitute the words goal, aim, purpose, meaning in the stead of object:
Staying late at the office to discuss shorter working hours rather defeats the purpose of the exercise.
Defeats the purpose is a very common combination of words.
What is the meaning of the whole sentence? Suppose an employee has proposed to gather a meeting (this is the exercise) in the office to discuss what could be done to make the working hours shorter. This is the purpose of the exercise: to make working hours shorter. But staying late at the office actually makes working hours longer.
So we have a contradiction: the proposed exercise defeats its own purpose.