Clip mask entities that are not part of a reference

I have file that I will be using at different scales. At the current scale, part of the background extends past the border (see screenshot below). When I try to clip mask using the inside border line, Unfortunately it wants to clip the stuff inside the border (which is more than likely what it was intended to do), while I need it to mask the stuff outside the border!

Sort of trimming and erasing (a practice I don't want to set a precedent for), can this be masked? What do I need to do?

Thanks!

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