Having a movie clip rotate back to its original position

1

I created a movie clip, an aircraft compass which rotates till the button is pressed but I would like it to return back to its original position in opposite direction of smooth rotation and stop. I already have the following script on the clip.

onClipEvent(enterframe) {
if(spin) {
_rotation -= 2;
}
}

and this on the button

on(Press) {
_root.marker.spin = 1;
}
on(Release) {
delete _root.marker.spin;
}

Can anyone help me.