On which playmates have their ups and downs (6)

I believe the answer is:

seesaw

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'ups and downs' is the definition.
(thesaurus)

I don't understand the remainder of the clue.

(Other definitions for seesaw that I've seen before include "wild mare" , "Plaything that's ridden up and down" , "Playground equipment for kids" , "source of imbalance" , "Playground balancing board" .)

I've seen this clue in the Evening Standard.

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