This is the problem I'm stumbling upon a lot recently. You start sculpting on perfectly flat surface, then you make some creases, maybe deeper creases, or maybe you are building geometry with inflate brush inside a hole like eye socket, but eventually this happens - you have a crease in your geometry that is so deep that no amount of smoothing or scraping is going to get rid of.
Actually when you look at it in edit mode it looks like a bubble of geometry that is stuck under the "skin" of your model. The only way to repair this place is to switch to edit mode and then just remove offending geometry vertex by vertex, then close the hole with some polygons or even one big n-gon and then you can sculpt in this area again.
My question is - is it possible to flatten or smooth (repair) such area quickly, with sculpting tools, without going out of sculpt mode?
Please take a look at these images:
The view from inside of the head, offending geometry looks like an air bubble: