after releasing the wires and having attached strings to retrieve them (if needed)
flip it around and remove the clamps, then flip it backwards and sideways, crush it using multi-grips, then align the short side of the pancaked box with the long axis of the hole and pull it through.
I learned a little about these doo dads this year when the two on our home started leaking after the winter freeze we had in Texas. The rubber grommets inside both went bad. After trying to get them off with no success, I took a picture and headed down to my favorite plumber dude on isle 9 at our local Home Depot. He laughed and smirked a little, then told ...
On this style of light fixture, you'll usually find that one of the three clips is spring loaded and can be pulled horizontally away from the center. This allows you d to remove the glass. There is no need to unscrew the clips.
If you're talking about the metal-colored screw that sticks out parallel to the ceiling, use a nut driver:
(image from Klein Tools via Google)
on this hexhead (not "flat head") fastener. They're available in many sizes, and you'll need one to fit this fastener head.
Props to @brhans in comments.