Final answer:
The phrase 'he was running down like an overwound clock' is a simile because it uses the word 'like' to make an indirect comparison between a person's state and an overwound clock winding down.
Step-by-step explanation:
The literary device exemplified in the phrase "he was running down like an overwound clock" is a simile. A simile is a figure of speech that makes a comparison, showing similarities between two different things, explicitly using the word as or like. In this case, the comparison of the person to an overwound clock using the word like fulfills the definition of a simile, as it indirectly compares the state of the person to the state of the clock winding down.