Is justice in a city really just for Plato? Why or why not?
This question appears to have in mind the ideal city, which is discussed by Socrates and some of his fellow Athenians in Plato's Republic.
This question strikes me as very open-ended and could have a number of different possible answers. First, I think the questioner needs to decide which Plato he/she means, the Plato whose own city put Socrates to death in 399 BCE, or the Plato who is concerned merely with the discussion that is unfolding in his Republic.
Also, the questioner needs to define what "city" he/she means. Is this the ideal city, the discussion of which is the focus of most of the Republic, or is this some other city?
If we assume that the questioner means the ideal city around which most of the discussion of the Republic revolves, then I would argue that justice in that ideal city probably cannot be just. My reason for this is based on the discussion in Republic 10 of things which are "thrice removed from the truth." Because things created by human beings who are not experts are thrice removed from the truth they cannot be perfect representations of a thing. True perfection only exists in the minds of the gods, as Socrates points out, and therefore any human creation, even something created by an expert, is not truly perfect. Therefore, any justice meted out by human beings cannot be truly just. Only the gods know what is truly just, as appears to be indicated in this quotation from Republic 10: "the nature both of the just and unjust is truly known to the gods" (Benjamin Jowett translation).
Thus, based on the argument about those things which are "thrice removed from the truth", I would argue that any justice crafted by human beings cannot be truly just for Plato.