Less than 7 hours (6 or less) is typically considered a part time arrangement. If you're a high performer, you can try to negotiate for this kind of situation, but it's rare to start a job with that type of benefit. You should expect to lose a proportionate amount of your salary, (keep in mind that depending on your tax bracket, the percentage hit to your take home will be less affected.)

The benefit of being officially on part time is that it sets everyone's expectations and shouldn't negatively impact your promotion trajectory, especially if you're able to fulfill your duties on a part time schedule.