I am not very sure if I understand your question. But if it says THR IDLE when AP is on then the way to reduce speed is to reduce rate of descent until reaching target speed then descending at planned rate again. If your question is how to slow down if it is throttle at idle power.
Depending on rate of descent the throttle may increase power again if the rate of descent is not high enough. The AP will automatically adjust rate of descent or climb to reach the target speed then climb or descend at the planned rate.
AP means Autopilot if you do not know.