And with her chin raised a little, as if she were balancing something on it which was quite likely to fall
Nick gets pretty specific with his description of Jordan and shows her attempts at a queen-like disposition. She isn’t actually balancing anything on her chin, but the upright way she carries herself makes it look like she is. This is an interesting way of saying that Jordan always is trying to keep things under control, or perhaps this is Jordan trying to balance her two lives: that of an emancipated successful sportsperson and the new (alien) life of a rich, young lady.
To help improve the meaning of these lyrics, visit “The Great Gatsby (Chapter I)” by F. Scott Fitzgerald and leave a comment on the lyrics box