Unpleasant article written about pen (5)
I believe the answer is:
nasty
'unpleasant' is the definition.
(nasty can mean unpleasant or horrible)
'article written about pen' is the wordplay.
'article' becomes 'an' (English indefinite article).
'written about' is a reversal indicator.
'pen' becomes 'sty' (both can mean an animal enclosure).
'an' back-to-front is 'na'.
'na'+'sty'='NASTY'
(Other definitions for nasty that I've seen before include "Awful" , "Filthy" , "rank" , "Repulsive" , "Disagreeable" .)