That's definitely crimped in, just like luker said. There's no way to tell exactly what size it should take from the photo, but I'd say luker's educated guess of 27.0, or the 26.6 mentioned earlier, or even 26.4mm would be possibilities. The only way to say for sure is to do as luker said earlier and get a caliper reading from further down inside the seat tube, where it would be unaffected by the crimping. I mean, I guess since you say it doesn't slip you could just ride it. Or you can carefully bend the clamp area back to shape and use a shim. Or you can get a caliper reading and get the right size post, which will also require carefully bending the clamp area back to shape.

Regardless, I stand corrected.