Toy that goes up and down (4)
(Other definitions for yoyo that I've seen before include "Plaything" , "Toy" , "Item on string" .)
(Other definitions for yo-yo that I've seen before include "Toy again in fashion" , "Toy that rises and falls" , "Toy consisting of a reel and string" , "-- - -- Ma, American cellist of Chinese/French origins" , "An up-and-down toy on a string" .)