Man City upset about the Italian's aggression (9)
I believe the answer is:
militancy
'aggression' is the definition.
(thesaurus)
'man city upset about the italian's' is the wordplay.
'upset' is an anagram indicator.
'about' indicates putting letters inside.
'the italian' becomes 'il' ('the' in Italian).
'man'+'city'='mancity'
'mancity' with letters rearranged gives 'mitancy'.
'mitancy' enclosing 'il' is 'MILITANCY'.
(Other definitions for militancy that I've seen before include "... violence" , "Activism" , "Confrontational behaviour" .)