Is it permissible to work for another company during official working hours, without the primary company's knowledge, given that there is a clause in the contract prohibiting it?

An employee is not permitted to use their free time during working hours to work for another entity without permission, whether the company explicitly states this prohibition or not. This is because the employee is a private contractor, and their benefit is owned solely by the employer for the entire duration of work. Furthermore, "Muslims are bound by their conditions [agreements]."