Posed wearing Selfridges' ultimate fabrics (6)
I believe the answer is:
satins
'fabrics' is the definition.
(satin is a kind of fabric)
'posed wearing selfridges' ultimate' is the wordplay.
'posed' becomes 'sat' ('sit' can be a synonym of 'pose').
'wearing' becomes 'in' (wear in (and 'in' is fashionable)).
'ultimate' suggests the final letters.
The final letter of 'selfridges' is 's'.
'sat'+'in'+'s'='SATINS'
(Other definitions for satins that I've seen before include "Stains on shiny cloths" , "Stains in expensive cloths" , "Stains on rich cloths" , "Stains on shiny fabrics" , "Gives smooth surface" .)