Fooling around and also hyper, possibly (9)
I believe the answer is:
horseplay
'fooling around' is the definition.
(both can mean silly behaviour)
'also hyper possibly' is the wordplay.
'possibly' indicates an anagram.
'also'+'hyper'='alsohyper'
'alsohyper' is an anagram of 'HORSEPLAY'.
'and' is the link.
(Other definitions for horseplay that I've seen before include "Messing about (as a steed?)" , "Rough boisterous action" , "Rough or boisterous activity" , "rough and tumble" , "Shop early (anag) - tomfoolery" .)