One opposing article returned it (4)
I believe the answer is:
anti
'one opposing' is the definition.
(prefix meaning 'opposing')
'article returned it' is the wordplay.
'article' becomes 'an' (indefinite article).
'returned' shows that the letters should be reversed in order.
'it' backwards is 'ti'.
'an'+'ti'='ANTI'
(Other definitions for anti that I've seen before include "Opposed to followers of Hitler" , "on the other side" , "Adversary" , "not favoured" , "Prefix meaning against" .)