Duplicate your clip and stack it on a layer above. Add Gussian Blur and a mask to the top layer. Set the mask to inverted and experiment with size, shape, the horizontal and vertical slider adjustments, and feathering.
You can also do this with the Vignette effect. Set the Darken slider to zero and dial up the blur value. The blur value can increase far beyond the slider range. Just double click the value box and scroll up on the mouse.