Knock off article covering one case of vice (6)
I believe the answer is:
thieve
'knock off' is the definition.
('thieve' can be a synonym of 'knock off')
'article covering one case of vice' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech**).
'covering' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'case of' says to hollow out the word (remove centre letters) (outsides of).
'vice' with its centre removed is 've'.
'the' placed around 'i' is 'thie'.
'thie'+'ve'='THIEVE'
(Other definitions for thieve that I've seen before include "Misappropriate" , "Nick things" , "Knock off" , "Pinch" , "Steal things" .)