Before this can be defined, we need to know the origins.
In one startrek episode there was a group called 'the circle' whose logo kept popping up everywhere. The circle are a group who reckon that only bajorans should be on bajor.
So if someone's a member of the circle, it means that they are really anti-immigration to the point where they might well be willing to build <country's> great wall, especially if there has been some past bad experience with people coming in and this stems from a slight to severe over-reaction to that past injustice.
Me: What are you doing?
RP: *starts lecturing me about immigration and how immigrants have no rights to be here and such.*
Me: You are such a member of the circle/You are such a circle/You are such a cm.
((DISCLAIMER: Random person does not represent my viewpoints - he merely represents the circle's beliefs.))