Thanks for your patience.
1) This clause is there usually in all Employment contracts. Employers cover themselves in case there is occasional additional work to be done , so employees are expected to do that without any overtime-pay .
Employees are not expected to work more than 48 hours per week, due to the working time regulations.
2) Notify your employer, that you would be working 4 days per week, and you would be free to undertake any other work in your off days.
3) If you can re-use the idea or method, would depend on your profession.
However, this is usual IP clause , which all employees have to agree to , which simply means that any IP created would belong to the employer.
If you need further assistance, please let me know.
Thank you and all the best.