The only way I know of is to mask the area then apply very weak Gaussian Blur FX to the area. I'd use a mask made as a png file in an image editor, using an image saved from the timeline as a guide. But if your subject is moving you have the complication of having to animate the masks position with keyframes.

Presumably you are familiar with using a mask with a copy of the Event in a Parent/Child track arrangement.

Edit: Oops I've just realised you are using MS15 Basic which doesn't have Parent / child compositing tracks. Sorry. Another reason for upgrading to Platinum.