Obtaining a higher degree (PhD or MD) is a requirement to secure most senior academic posts.
Unlike most clinical academic positions, a PhD fellowship allows you to devote your time 100% to your academic studies, usually for a period of three years. At the end of this time there is an expectation that you will submit a PhD thesis.
There is no requirement for a Clinical Fellow undertaking a PhD to have had any other academic related positions. However most Fellowships are awarded directly to an applicant on the basis of a specified project as well as on a strong CV. It is usual for clinicians to have spent time within a research environment developing a specific research project in conjunction with their supervisor in order to secure funding.
This is the advantage of having been an ACF as it allows for time out of the clinical environment in order to formulate an appropriate project whilst research contacts are being made.