One charged expert to pen last article (9)
I believe the answer is:
defendant
'one charged' is the definition.
(I've seen this before)
'expert to pen last article' is the wordplay.
'expert' becomes 'deft' (I've seen this in another clue).
'to pen' is an insertion indicator.
'last' becomes 'end' (I've seen this in another clue).
'article' becomes 'an' (indefinite article).
'end'+'an'='endan'
'deft' going around 'endan' is 'DEFENDANT'.
(Other definitions for defendant that I've seen before include "The one before the court" , "One charged" , "Against whom an action is brought in a law court" , "Person accused" .)