Fake article possessed by Nitsy upset Conservative (9)
I believe the answer is:
synthetic
'fake' is the definition.
(I've seen this before)
'article possessed by nitsy upset conservative' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech).
'possessed by' is an insertion indicator.
'upset' indicates anagramming the letters.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'nitsy' anagrammed gives 'synti'.
'the' inserted into 'synti' is 'syntheti'.
'syntheti'+'c'='SYNTHETIC'
(Other definitions for synthetic that I've seen before include "Artificially produced" , "Not natural in origin" , "Man-made (fibre)" , "Artificially made" , "Mock" .)