Post Office sure upset pretentious type (6)
I believe the answer is:
poseur
'pretentious type' is the definition.
(I've seen this before)
'post office sure upset' is the wordplay.
'post office' becomes 'po'.
'upset' indicates anagramming the letters.
'sure' with letters rearranged gives 'seur'.
'po'+'seur'='POSEUR'
(Other definitions for poseur that I've seen before include "Uprose (anag.)" , "Pretentious type" , "One seeking to impress others with affected style" , "One trying to impress others" , "Hypocrite!" .)