Witness cops treat badly (9)
I believe the answer is:
spectator
'witness' is the definition.
(synonyms)
'cops treat badly' is the wordplay.
'badly' indicates an anagram (letters in the wrong order).
'cops'+'treat'='copstreat'
'copstreat' is an anagram of 'SPECTATOR'.
(Other definitions for spectator that I've seen before include "I attend sporting event" , "Part of gate" , "Onlooker at game" , "Looker-on" , "One attends match" .)