One taking the law into his own hands leaving IT in disarray (9)
I believe the answer is:
vigilante
'one taking the law into his own hands' is the definition.
(I've seen this before)
'leaving it in disarray' is the wordplay.
'in disarray' indicates anagramming the letters.
'leaving'+'it'='leavingit'
'leavingit' is an anagram of 'VIGILANTE'.
(Other definitions for vigilante that I've seen before include "Self-appointed law enforcer" , "Unauthorised law enforcer" , "Policeman unofficially" , "Person in group taking the law into its own hands" , "Someone taking the law into their own hands" .)