Auction includes British fur (5)
I believe the answer is:
sable
'fur' is the definition.
(sable is a kind of fur)
'auction includes british' is the wordplay.
'auction' becomes 'sale' (type of sale).
'includes' indicates putting letters inside.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'sale' enclosing 'b' is 'SABLE'.
(Other definitions for sable that I've seen before include "Bales of precious fur" , "Adventurous courage" , "Bales of rich fur" , "fur coat" , "Fur from member of weasel family" .)