When looking at my objects in 3d view I'm seeing a gray cone that seems to be hiding other objects.
Can anyone help me turn this off and explain what it is please.
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You somehow enabled the clipping border. It's created in 2D, but projects into 3D, thus it looks like a cone from certain angles.
Disable it by clicking View > Clipping Border... in 3D View header, or press AltB