CAS = Dynamic Pressure
Dynamic Pressure = 1/2pV^2(where V = TAS)
Density is increasing but CAS is staying the same so TAS must decrease

Try putting some basic numbers in to illustrate (the numbers themselves are irrelevant for this purpose).

Dynamic Pressure = 4 x 10^2 = 400 - so CAS now needs to stay at 400

Pressure increases so density increases so:-

Dynamic Pressure = 8 x 10^2 = 800 - with TAS constant in this example CAS would increase therefore for the CAS to stay constant the TAS would need to reduce.