Article concerning the unknown (4)
I believe the answer is:
anon
'the unknown' is the definition.
(I've seen this before)
'article concerning' is the wordplay.
'article' becomes 'an' (indefinite article).
'concerning' becomes 'on' (both can mean 'in relation to').
'an'+'on'='ANON'
(Other definitions for anon that I've seen before include "Not identified by name" , "Mystery writer" , "devoid of credit" , "Soon; unnamed" , "In due course" .)