Transaction includes black fur (5)
I believe the answer is:
sable
'fur' is the definition.
(sable is a kind of fur)
'transaction includes black' is the wordplay.
'transaction' becomes 'sale' (sales are a type of transaction).
'includes' means one lot of letters goes inside another.
'black' becomes 'b' (as in pencil classifications).
'sale' placed around 'b' is 'SABLE'.
(Other definitions for sable that I've seen before include "Fur from member of weasel family" , "Type of fur" , "Dark brown - black" , "Bales of rich fur" , "Bales (anag.)" .)