Piece of pretentiousness by administrative subdivision (7)
I believe the answer is:
sidearm
'piece of pretentiousness' is the definition.
'sidearm' can be an answer for 'piece' (I've seen this before). I'm unsure of the 'of pretentiousness' bit.
'by administrative subdivision' is the wordplay.
'by' becomes 'side' (synonyms).
'administrative subdivision' becomes 'arm' (synonyms. I am not sure about the 'administrative' bit.).
'side'+'arm'='SIDEARM'
Can you help me to learn more?
(Other definitions for sidearm that I've seen before include "Piece" , "Officer may carry this" , "weapon" , "One may be fired" .)