Unfinished article is not that (4)
I believe the answer is:
this
'not that' is the definition.
(this one as opposed to that one)
'unfinished article is' is the wordplay.
'unfinished' means to remove the last letter.
'article' becomes 'the' (the definite article).
'the' with its final letter taken off is 'th'.
'th'+'is'='THIS'
(Other definitions for this that I've seen before include "The thing here referred to" , "The thing just mentioned" , "Pronoun to point out nearby things" , "What's here" , "Demonstrative pronoun" .)