Horseplay (10)
I believe the answer is:
tomfoolery
'horseplay' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tomfoolery that I've seen before include "Acting stupidly" , "Silliness" , "Craziness" , "Silly behaviour" , "Shenanigans" .)