Socially unacceptable foreign noun (3-1)
I believe the answer is:
non-u
'socially unacceptable' is the definition.
(I've seen this before)
'foreign noun' is the wordplay.
'foreign' is an anagram indicator (foreign order of the letters).
'noun' with letters rearranged gives 'NON-U'.
(Other definitions for non-u that I've seen before include "Common - noun (anag)" , "Uncultured" , "Plebeian" , "Not socially acceptable" , "Socially naff" .)