Final answer:
In the context of the poem, 'derivative' most likely means unoriginal, indicating a lack of innovation in poetry that leads to unintelligibility.
Step-by-step explanation:
Based on the context of the excerpt from "Poetry," the most likely definition for “derivative” is unoriginal. The passage suggests that poetry becomes less admirable and more difficult to understand when it lacks originality and begins to imitate other works too closely. This mirrors the broader experience with language and meaning, where original and concrete words evolve into more abstract terms through usage over time. Derivative here connotes a lack of innovation, uniqueness, and freshness, resulting in poetry or language that is not engaging or comprehensible.