Stockings plugged by posh fashion company (5)
I believe the answer is:
house
'fashion company' is the definition.
(fashion house)
'stockings plugged by posh' is the wordplay.
'stockings' becomes 'hose' (I've seen this before).
'plugged by' means one lot of letters goes inside another (plug can mean to block up a gap).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'hose' enclosing 'u' is 'HOUSE'.
(Other definitions for house that I've seen before include "Dwelling or official assembly" , "Dwelling-place" , "Building for dwelling in" , "Provide with shelter" , "Residence, dwelling" .)