It goes back to article expressing opposition (4)
I believe the answer is:
anti
'opposition' is the definition.
(I've seen this before)
'it goes back to article' is the wordplay.
'goes back' says the letters should be written in reverse.
'to' says to put letters next to each other (I've seen this in other clues).
'article' becomes 'an' (English indefinite article).
'it' back-to-front is 'ti'.
'ti' put after 'an' is 'ANTI'.
'expressing' acts as a link.
(Other definitions for anti that I've seen before include "Negative vote" , "on the other side" , "An opponent - opposed to party policy, etc" , "Hostile" , "Policy's opponent" .)