Wimmel is correct in his comment. You need to change tileset*;
to &tileset;
This will return a pointer to your tileset
EDIT:
should be return &(const sf::Texture)tileset;
but I'm not sure if that's what you want. I'm not a C++ expert and const correctness is not my strong suit, so I might do some more research on const correctness just to make sure that's what you really want or not.