Stuns room, misbehaving: outrageous (9)
I believe the answer is:
monstrous
'outrageous' is the definition.
(thesaurus)
'stuns room misbehaving' is the wordplay.
'misbehaving' is an anagram indicator.
'stuns'+'room'='stunsroom'
'stunsroom' with letters rearranged gives 'MONSTROUS'.
(Other definitions for monstrous that I've seen before include "Extraordinarily large" , "Terrible" , "Outrageously wrong" , "Outrageously evil" , "really good!" .)