Black market transaction interrupted by bishop (5)
I believe the answer is:
sable
'black' is the definition.
(heraldic term for black)
'market transaction interrupted by bishop' is the wordplay.
'market transaction' becomes 'sale' (I've seen this in another clue).
'interrupted by' means one lot of letters goes inside another.
'bishop' becomes 'B' (abbreviation for bishop in chess).
'sale' placed around 'b' is 'SABLE'.
(Other definitions for sable that I've seen before include "Fur - heraldic black" , "fur coat" , "Brush" , "Adventurous courage" , "Bales (anag.)" .)