Had this in the workshop last week. Customer bought the bike second hand a few months ago like this. The previous owner had worn the seat sleeve and had overtightened the clamp to compensate, and then cut the clamp bolt down and fitted a recessed nut from a brake instead of the usual nut, and for some reason managed to fit it upside down!
The clamp area was stressed, and the tube was slightly ovalised. It took me an age to manipulate the clamp back into shape and then carefully open the tube up and ream it out to get it back into shape. Even then the new seat sleeve took LOTS of reaming to get it back to 31.8mm where the seat post would slide nicely. A new rear clip finished the job off.
I was pleased to be able to save the frame, but the moral of the story is keep an eye on the seat sleeve and don’t overtighten the clamp!!
by VisibleOtter