A long gap since your first degree is a disadvantage, particularly in the absence of evidence of continued academic interests. Try to get some frank feedback from people who know your work well. Additional degrees are by no means the norm among applicants. Some obtain a masters degree to enhance their application and there is no doubt that a relevant qualification provides further evidence of your academic and research ability. Make sure that you are happy with the content of a Master s course before accepting a place.
Clinical experience is still essential.
An additional degree will not compensate for a first degree which is lower than a 2.2. Completing a PhD just to apply for clinical psychology is a long route into the profession. You should be interested in the work for its own sake, and perhaps be considering an academic career, to maintain your interest in it over the long three years it will take to complete. You would still need to have some relevant clinical experience and you would not be allowed to enter the course before the PhD was completed. There is no doubt however that a PhD provides evidence of your academic ability and perhaps will open other doors if the application to clinical doesn't work out.