Children play with this yo-yo (6)

I believe the answer is:

seesaw

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'yo-yo' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for seesaw that I've seen before include "Plaything with up-and-down movement" , "Playground device" , "Fluctuate" , "'Move up and down alternately'" , "source of imbalance" .)

I've seen this clue in The Guardian.

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