If there is such a clause, then I would think you are liable to pay as indicated.
And since this is a forum, I would also like to take this opportunity to comment that it has sadly become such a rampant practice nowadays for some people to just take up offers without seriously thinking over what they are going into.
Maybe you have a very good reason for changing your mind but do understand and see from the point of the employer. They go through a very tedious process of recruiting which probably includes advertising, vetting through candidates, putting the effort to call and arrange the interview appointment (and in this course, the interviewer actually dedicated and put aside some of their precious time for the interview). After shortlisting the candidate, probably a second round of interview is arranged, otherwise if the employment is confirmed, Appointment Letter is typed, record is put down and lots of other stuff like preparing keys, work station, cards, manuals, etc....is done.
After all these, you can imagine the frustration when the soon-to-be employee suddenly announce that he/she no longer wish to commence work.
Do not misunderstand me as this is not a reprimand response to Tan but just wish to share "the other side of the story" to those who might genuinely not aware of the preparation work done for a new employee.