They go up and down on a playground (7)

I believe the answer is:

seesaws

(Other definitions for seesaws that I've seen before include "Boards balanced on fulcrums, children's playthings" , "Playground games" , "Yo-yos" , "Pieces of playground furniture" , "Playthings in the park" .)

I've seen this clue in The New York Times.

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