Article upset Italian opponent (4)
I believe the answer is:
anti
'opponent' is the definition.
(I've seen this before)
'article upset italian' is the wordplay.
'article' becomes 'an' (indefinite article).
'upset' is a reversal indicator.
'italian' becomes 'it'.
'it' backwards is 'ti'.
'an'+'ti'='ANTI'
(Other definitions for anti that I've seen before include "(Person who is) not in favour" , "Opposed to followers of Hitler" , "Definitely not pro" , "[NO MAN]" , "on the other side" .)