If your on core clerkships the answer is that is what you do. Research is more icing on the cake. You need to excell in your clinical years. Remember, your a doctor first and a radiation oncologist second (everyone will say that on interviews). If you have time retrospective studies are good, but thats honestly a big if during most 3rd year rotations. You can always take more time to get research. You only get one shot at core clearkships.