Theme park feature’s period of ups and downs (6-7)

I believe the answer is:

roller-coaster

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'theme park' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's a link between them I don't understand?

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Another definition for roller-coaster that I've seen is " Fairground ride".)

I've seen this clue in The Times.

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