Black is black in retail event (5)
I believe the answer is:
sable
'black' is the definition.
(heraldic term for black)
'black in retail event' is the wordplay.
'black' becomes 'b' (abbreviation as in pencil classifications).
'in' means one lot of letters goes inside another.
'retail event' becomes 'sale' (I've seen this in another clue).
'b' going into 'sale' is 'SABLE'.
'is' is the link.
(Other definitions for sable that I've seen before include "Literary black" , "Bales of precious fur" , "Black; antelope" , "Black (heraldry); fur" , "Marten's fur - dark black" .)