Knowledgeable, fine article penned by socialist (4-4)
I believe the answer is:
well-read
'knowledgeable' is the definition.
(synonyms)
'fine article penned by socialist' is the wordplay.
'fine' becomes 'well' (similar in meaning).
'article' becomes 'a' (the indefinite article in English).
'penned by' means one lot of letters goes inside another (inserted letters are penned in).
'socialist' becomes 'red' (socialism is often associated with the colour red).
'a' going into 'red' is 'read'.
'well'+'read'='WELL-READ'
(Other definitions for well-read that I've seen before include "Knowledgeable on books" , "Highly literate" , "Scholarly" , "Erudite" , "Having good book knowledge" .)