Posh, boring models express disapproval (3)
I believe the answer is:
tut
'express disapproval' is the definition.
(tutting is making a disapproving sound)
'posh boring models' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'boring' means one lot of letters goes inside another (to bore something can mean to drill into it).
'models' becomes 'tt' (two T's as in the Ford Model T car).
'u' going into 'tt' is 'TUT'.
(Other definitions for tut that I've seen before include "Sound of mild disapproval" , "former ruler, in brief" , "I disapprove" , "Exclamation of disapproval or annoyance" , "King wrapped up" .)