Based on the context and the definition, the word 'derivative' makes reference to something that is not the result of an original idea, but something that has been based on something else.
In this excerpt from "Poetry" by Marianne Moore, 'derivative' is used as an adjective in order to describe "things". 'Derivative' is used to refer to something that does not offer new ideas because it has been developed from something else. It has a negative connotation since it is used to criticize something. Moreover, in this excerpt, it is highly connected to the idea of unintelligibility.