Cropping a dwg file

I have a 3d cad file which I have imported and all the levels have come through perfectly to give a 3d model of the site in Kubla Cubed.
The dwg file covers an area which I need to split into 3 separate sites so that quantities can be worked out separately for each site.

I have tried to crop the dwg file but after doing so, the 3d model still appears and I only want to see part of it for each site.
Is it possible to crop to just the area of the file that I need?

Screen shot included to show the area needed for 1 isolated part of the site.
The remaining area will form part of another project so this will need cropping too

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