Ayelat,
Clarizen does not have this functionality out of the box but our advanced customization engine is capable of doing just about anything which would include creating a workflow that will schedule (set Start Date) according to a Priority (Custom field). This is considered advanced configurations and I suggest either reaching out to your Customer Success Manager (if you have one) or joining one of our free Advanced Q&A Webinars were you are able to ask questions of one of our qualified Customer Success Managers.
Sincerely,
Boris