Postscript written about ill-mannered prigs (6)
I believe the answer is:
prudes
'prigs' is the definition.
(I've seen this in another clue)
'postscript written about ill-mannered' is the wordplay.
'postscript' becomes 'ps' (abbreviation).
'written about' is an insertion indicator.
'ill-mannered' becomes 'rude' (synonyms).
'ps' placed around 'rude' is 'PRUDES'.
(Other definitions for prudes that I've seen before include "Stuffy types" , "They are easily shocked by matters relating to sex" , "People with exaggerated sense of propriety" , "They're excessively concerned about propriety and decorum" , "Prim or priggish people" .)