"Line" on edge of clipped view on export

[cubswinn]

The attached .xar file is a sample button. When I select all and clip then export the attached .png file is the result. My problem is that I inevitably get a 1px wide discoloration on the edge of the .png image. This looks sloppy in the final web site since this button is over a background which matches the button background. Is there a setting or technique to deal with this issue.
thanks as always,
John