Express disapproval over posh ballerina’s skirt (4)
I believe the answer is:
tutu
'ballerina's skirt' is the definition.
(I've seen this before)
'express disapproval over posh' is the wordplay.
'express disapproval over' becomes 'tut' (I've seen this in other clues).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'tut'+'u'='TUTU'
(Other definitions for tutu that I've seen before include "Ballet dancer's costume" , "Ballerina's costume" , "Ballet dress" , "very short garment" , "North African nation" .)