This person's posturing as impressive (8)
I believe the answer is:
imposing
'impressive' is the definition.
('imposing' can be similar in meaning to 'impressive')
'this person's posturing' is the wordplay.
'this person's' becomes 'im' (this person means the crossword setter themselves).
'posturing' becomes 'posing' ('pose' can be a synonym of 'posture').
'im'+'posing'='IMPOSING'
'as' is the link.
(Other definitions for imposing that I've seen before include "Majestic" , "Impressive - or putting tax on" , "Dictating; impressive" , "Enormous - levying - striking" , "Taking liberties" .)