The surest fashion colour? (6)
I believe the answer is:
russet
'colour?' is the definition.
(I know that russet is a colour)
'the surest fashion' is the wordplay.
'fashion' indicates anagramming the letters.
'surest' anagrammed gives 'RUSSET'.
(Other definitions for russet that I've seen before include "Reddish-brown - not orange!" , "Surest type of colour" , "Reddish brown; apple" , "Autumnal red as of leaves" , "browny-red" .)