Appropriate article that is very penetrating (6)
I believe the answer is:
thieve
'appropriate' is the definition.
(I've seen this before)
'article that is very penetrating' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'very' becomes 'v' (abbreviation).
'penetrating' is an insertion indicator.
'ie'+'v'='iev'
'the' enclosing 'iev' is 'THIEVE'.
(Other definitions for thieve that I've seen before include "Take illegally" , "Pinch" , "Pilfer" , "Misappropriate" , "Knock off" .)