Over and over it goes (9)

I believe the answer is:

ferryboat

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'over' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Another definition for ferryboat that I've seen is " Charon's river craft?".)

I've seen this clue in The Globe and Mail.

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