Imagine that clearing the terrain region will leave behind a cube shaped hole, is it possible?
And btw, is it also possible to specify the materials inside the region? And deleting those materials only too?
I hope you understand, ask me if you don't.
https://developer.roblox.com/api-reference/function/Terrain
workspace.Terrain:FillRegion(Region3, resolution, 1792)
Region3 would be the region you want to change, resolution would be how smooth you want it, and 1792 is the material code for air
I'm not going to write it out for you anymore after this because you should have attempted it before coming here.
If this gets you going in the right direction, then please mark this as the accepted answer.