Black bishop captured in stratagem to move more material (5)
I believe the answer is:
sable
'black' is the definition.
(heraldic term for black)
'bishop captured in stratagem to move more material' is the wordplay.
'bishop' becomes 'b' (abbreviation used in chess).
'captured in' means one lot of letters goes inside another.
'stratagem to move more material' becomes 'sale' (move can mean to sell).
'b' put into 'sale' is 'SABLE'.
(Other definitions for sable that I've seen before include "Bales of rich fur" , "Bales (anag.)" , "Brush" , "Bales of valuable Russian fur" , "Dark brown - black" .)