Article penned by expert really is dull (7)
I believe the answer is:
prosaic
'dull' is the definition.
(similar in meaning)
'article penned by expert really' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'penned by' is an insertion indicator.
'expert' becomes 'pro' (short for professional).
'really' becomes 'sic' (shows a quoted error really appears in the source).
'pro'+'sic'='prosic'
'a' placed into 'prosic' is 'PROSAIC'.
'is' acts as a link.
(Other definitions for prosaic that I've seen before include "Commonplace, unromantic" , "So Capri! (anag)" , "Lacking poetry" , "Dull - pedestrian" , "Mundane" .)