Fooling around in Equus perhaps? (9)
I believe the answer is:
horseplay
I believe this is a double definition.
'fooling around' is the first definition.
(both can mean silly behaviour)
'equus perhaps?' is the second definition.
(Equus is a play partly about horses)
'in' is the link.
(Other definitions for horseplay that I've seen before include "Rough boisterous behaviour" , "Boisterous rowdy recreation" , "Knockabout" , "Rowdy shenanigans" , "High jinks" .)