Like good guy arresting city mugger (9)
I believe the answer is:
assailant
'mugger' is the definition.
(I've seen this before)
'like good guy arresting city' is the wordplay.
'like' becomes 'as'.
'good guy' becomes 'saint' (a saint is a virtuous person).
'arresting' is an insertion indicator.
'city' becomes 'LA' (Los Angeles).
'saint' going around 'la' is 'sailant'.
'as'+'sailant'='ASSAILANT'
(Other definitions for assailant that I've seen before include "One attacking another" , "Attacking person" , "mugger, say" , "who is on the offensive?" , "One attacks" .)