Egyptian monarch with posh skirt (4)
I believe the answer is:
tutu
'skirt' is the definition.
(I know that tutu is a type of skirt)
'egyptian monarch with posh' is the wordplay.
'egyptian monarch' becomes 'tut' (Tutankhamun, the ancient Egyptian pharaoh).
'with' says to put letters next to each other.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'tut'+'u'='TUTU'
(Other definitions for tutu that I've seen before include "Short dress" , "Giselle's skirt" , "South African archbishop, b. 1931" , "In which dancer may appear" , "North African nation" .)