Opposed to article leading to mother’s ruin (4)
I believe the answer is:
agin
'opposed to' is the definition.
(dialect form of against)
'article leading to mother's ruin' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'leading to' says to put letters next to each other (some letters go before or lead others).
'mother's ruin' becomes 'gin' (informal term for gin).
'a'+'gin'='AGIN'
(Other definitions for agin that I've seen before include "Not in favour of" , "Against (facetious)" , "Opposed to local" , "Dialect form of against" , "Hostile to" .)