import ur clip in premiere, then in the title, make a circle which is white color, make it as big as u want, then save it, put that circle mask on top of ur clip, then, go to the keying folder, u will see a key called tracke matte, put that key on the clip of ur video, the go in the effects properties, and in the track matte properties, make the matte to which ever layer ur circle mask is in, and make the circle mask layer invisible by clicking the eye ball, i hope that helps.