Check out the Blend Modes example. editor.construct.net
The one you'll be interested in is Source In. To set up, the mask and the map should be on the same layer, with the force own texture property checked. The mask should be at the bottom of the layer, while the map is on top. The mask has no blending option set, and the map is Source In. The filled in opaque of the mask are what will show, and the transparent areas will not be visible.
You will move your map sprite object according to how the player moves, scaled to however your minimap scale is compared to your actual layout. Alternatively you might be able to do it with parallax settings on the minimap layer.