Final answer:
The simile in the sentence indicates that the yard was not well maintained, resembling an overgrown hayfield due to its lengthy grass.
Step-by-step explanation:
The simile used in the sentence "The neighbor's yard was like a hayfield when he finally got around to mowing it." is to suggest that the yard was not well maintained. This comparison metaphorically describes the yard's overgrown state by likening it to a hayfield, which is typically a field of long grass that is intended to be mowed and dried for use as fodder. In this context, it implies that the grass in the yard had grown so long that it resembled a hayfield, thereby needing a much overdue mowing.