The writer is appearing in knockout garment with sash (6)
I believe the answer is:
kimono
'garment with sash' is the definition.
(type of Japanese garment)
'the writer is appearing in knockout' is the wordplay.
'the writer is' becomes 'I'm' ('I am', referring to the person who wrote the clue).
'appearing' becomes 'on' (e.g. on stage or on television).
'in' means one lot of letters goes inside another.
'knockout' becomes 'KO' (abbreviation).
'im'+'on'='imon'
'imon' put within 'ko' is 'KIMONO'.
(Other definitions for kimono that I've seen before include "Eastern robe" , "Traditional Japanese robe with wide sleeves" , "Long loose Japanese robe" , "Japanese-style dressing gown or robe" , "Japanese clobber" .)