What term describes a fanciful, particularly clever extended metaphor?

The term that describes a fanciful, particularly clever extended metaphor is "conception." A conceit takes an idea or metaphor and elaborates on it throughout a poem or prose piece, connecting disparate elements in surprising and imaginative ways. This allows the writer to draw intricate comparisons that extend beyond the initial metaphor, often creating a richer meaning or deeper understanding of the themes being explored.

Conceits are especially prominent in metaphysical poetry, where they often juxtapose complex and abstract ideas with vivid imagery, engaging the reader's imagination. The cleverness lies in the unexpected connections made between seemingly unrelated subjects, prompting readers to see them in a new light.

In contrast, imagery pertains to the use of descriptive language to evoke sensory experiences, while simile specifically compares two different things using "like" or "as." Analogy involves similarities between two concepts but does not necessarily have the extended or elaborate nature that characterizes a conceit. Therefore, conceit is the term that most accurately captures the essence of a clever, extended metaphor.