Writer's counterpart penning article (5)
I believe the answer is:
twain
'writer's' is the definition.
(author Mark Twain)
'counterpart penning article' is the wordplay.
'counterpart' becomes 'twin' (synonyms).
'penning' indicates putting letters inside.
'article' becomes 'a' (English indefinite article).
'twin' going around 'a' is 'TWAIN'.
(Other definitions for twain that I've seen before include "A number in company" , "Mark ___ , American writer" , "Old pair beginning" , "brace" , "Kipling never expected to meet" .)