Can I cut a piece of newel post base sticking out of slanted ceiling?
I would not. It was probably left long like that because the attachments (bolts, screws, nails) are right where the slant is, so if you cut off the tail piece, those fasteners are now too close to the end of the wood and will break out.

I would just create a larger box around that post end, going all the way across to the wall, so that it just looks neater.