If you mean IFR VMC descent through other IFR traffic as defined in the MATS 1, (radar not available, one pilot able to see the other, etc.), then you can do that subject to those conditions. It has nothing to do with terrain separation as it must happen above MSA. Unless, of course, you are doing a visual approach, which requires the pilot to have visual reference with the gorund before descending below MSA.
So in the scenario I think you mean: Providing all the criteria for a VMC descent through other IFR traffic are met, you may clear IFR traffic to descend visually though other IFR traffic in order to make a Visual Approach, provided he does not descend below MSA without visual reference to the ground.
Remember that a Visual Approach is still subject to the IFR rules and must be provided with the requisite separation from necessary traffic depending upon the class of airspace. The responsibility for not impacting the ground during the approach rests with the pilot!