Article about unknown writer (4)
I believe the answer is:
anon
'unknown writer' is the definition.
(short for an anonymous author)
'article about' is the wordplay.
'article' becomes 'an' (English indefinite article).
'about' becomes 'on' (as in a book 'on' a certain topic).
'an'+'on'='ANON'
(Other definitions for anon that I've seen before include "devoid of credit" , "Not identified by name" , "Well-published writer" , "Presently - unattributed" , "Nameless (abbr.); soon" .)