Black market stocks book (5)
I believe the answer is:
sable
'black' is the definition.
(heraldic term for black)
'market stocks book' is the wordplay.
'market' becomes 'sale' (I've seen this in other clues).
'stocks' indicates putting letters inside (stocks can mean carries or contains).
'book' becomes 'b' (abbreviation for book).
'sale' enclosing 'b' is 'SABLE'.
(Other definitions for sable that I've seen before include "Black; antelope" , "Brush" , "Fur from member of weasel family" , "fur coat" , "Dark brown fur of the marten" .)