Transaction includes British fur (5)
I believe the answer is:
sable
'fur' is the definition.
(I know that sable is a fur)
'transaction includes british' is the wordplay.
'transaction' becomes 'sale' (sales are a type of transaction).
'includes' means one lot of letters goes inside another.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'sale' going around 'b' is 'SABLE'.
(Other definitions for sable that I've seen before include "Literary black" , "Marten's fur - dark black" , "Type of fur" , "Dark brown - black" , "Bales of rich fur" .)