He thinks he can get Exley to go along with a phony story by promising to advance his career.
The dialogue is pretty clear in this scene. Smith thinks that the uncompromising Bud White is dead and tells Exley that if goes along with what he intends to tell police (i.e., that Smith and Exley were working together to kill all the bad guys) then he will shoot Exley straight to the top. Throughout the film, Exley is seen as a careerist who looks out for his own advancement first and foremost. Smith is banking on this reputation that Exley will do the wrong thing and back up his story. Instead, Exley shows himself to be a man of integrity and kills Smith.