Arresting idiot is utter common sense (6)
I believe the answer is:
sanity
'common sense' is the definition.
(thesaurus)
'arresting idiot is utter' is the wordplay.
'arresting' means one lot of letters goes inside another.
'idiot' becomes 'nit' (nit can mean a stupid person).
'is utter' becomes 'say' (to utter something is to speak or say it).
'nit' going inside 'say' is 'SANITY'.
(Other definitions for sanity that I've seen before include "Rational condition" , "Condition of having a sound mind" , "Mental wellness" , "Rationality" , "presence of mind?" .)