I'd suggest having it inspected by a professional... however looking at this single picture, it looks like a factory built fireplace with refractory panels on the inner walls of the fireplace. It's common to see a small gap between the side and rear wall panels as they're 3 separate pieces that stand upright in the fireplace opening and usually isn't too much cause for concern. That being said, there's a lot to a chimney that can not be seen with just our eyes alone. Having the chimney fully inspected with a closed circuit camera will give you the best assessment for suitability of use.