Could you maybe move the model a little further away or describe in more detail what you want to accomplish? Without knowing what you’re sculpting, it’s hard to offer any suggestions.
But I’ll take a wild guess here. Your model is titled, “new raptor sculpt”, which I take to mean the dinosaur. Most dinosaurs have this thin membrane where the “cheek” would be, and both the upper and lower jaws clearly defined, since they don’t have much underlying tissue or fat, if any.

Personally, this is what I’d probably do:
1.Sculpt a second lower jaw as a separate subtool.
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Overlay it over the main model’s lower jaw and “dig into” the original jaw using the Zsub of any clay brush to get rid of the original lower jaw
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Merge the lower jaw to the main model, and smooth out the differences in the connecting areas between the lower jaw and main model using the necessary brushes.
Just my 2 cents’ worth, and there are many ZBrush experts here with much more experience, so you might want to wait for some better advice.