Stunner going about one day in gown with sash (6)
I believe the answer is:
kimono
'gown with sash' is the definition.
(type of Japanese garment)
'stunner going about one day' is the wordplay.
'stunner' becomes 'ko' (I've seen this in other clues).
'going about' means one lot of letters goes inside another.
'one' becomes 'I' (Roman numeral).
'day' becomes 'mon' (abbreviation for Monday).
'i'+'mon'='imon'
'ko' placed around 'imon' is 'KIMONO'.
'in' is the link.
(Other definitions for kimono that I've seen before include "dressing-gown" , "Woman's loose robe, tied with a sash" , "Japanese attireimoni" , "Japanese loose, sashed garment" , "Japanese wrap" .)