Opposition member’s article Congress backed (4)
I believe the answer is:
anti
'opposition member's' is the definition.
(I've seen this before)
'article congress backed' is the wordplay.
'article' becomes 'an' (indefinite article).
'congress' becomes 'it' (congress can mean sex).
'backed' 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 "Not in favor of" , "Policy's opponent" , "An opponent - opposed to party policy, etc" , "'No' voter" , "Prefix meaning against" .)