This is quite easy by using the inverted polygon renderer - it simply renders the outside of a polygon with a fill color instead of the standard inside fill.
I suppose you would like to clip your map content to some kind of region defined by a polygon. You could create this region polygon either from your existing data or edit one yourself. Then you choose the inverted-polygon renderer in the symbology tab in the layer properties or styling panel (access via F7) and set the fill color to white (or whatever color the surrounding map should have).
Screenshot using grey color is provided by this answer
You can also adjust which layers are excluded in the 'clip' by moving the inverted polygon layer up or down in your collection of layers.