Show us a pic. Carbon is very easy to repair despite what you may hear. You can sand the area down and open up the crack a bit to allow Epoxy to fill the gap and then wrap a few twists of wet out carbon tape over the clamp area. Wrap the area with electrical tape to compress the repair. When cured use a slightly larger seatpost clamp.
You should insert a seatpost of a slightly smaller size and cover it with clear tape until a slide fit is achieved. That will keep the interior clean and you'll be able to twist the post out even if traces of Epoxy leak into the seattube.
You will have to re-cut the slot and tidy the area up a bit and give it a final coat of Epoxy to blend everything.