Basically there is a big image inside the circle ‘canvas’. Now there isnt a problem if I use a square, but I have no idea if its possible with another shape (using directScrolledFrame)
I suggest blending a mask image on top of the minimap which would judging from your image be a white circle on a black background.
It gets a bit more tricky if you use a display region (and not render to texture) as minimap, but it surely is possible using some alpha/stencil-test trickery or so.