Dull article, one penned by prisoner (9)
I believe the answer is:
inanimate
'dull' is the definition.
(I've seen this before)
'article one penned by prisoner' is the wordplay.
'article' becomes 'an' (indefinite article).
'one' becomes 'i' (Roman numeral).
'penned by' means one lot of letters goes inside another.
'prisoner' becomes 'inmate' (inmate is a kind of prisoner**).
'an'+'i'='ani'
'ani' put within 'inmate' is 'INANIMATE'.
(Other definitions for inanimate that I've seen before include "wooden" , "Showing no feeling" , "spiritless" , "dead" , "Without life" .)