Improper noun, not socially acceptable (3-1)
I believe the answer is:
non-u
'not socially acceptable' is the definition.
(I've seen this before)
'improper noun' is the wordplay.
'improper' indicates an anagram.
'noun' with letters rearranged gives 'NON-U'.
(Other definitions for non-u that I've seen before include "Not characteristic of the upper classes" , "Not socially acceptable" , "not liked by the toffs" , "Lower class" , "Lower-class" .)