Black market grips Britain (5)
I believe the answer is:
sable
'black' is the definition.
(sable is a kind of black)
'market grips britain' is the wordplay.
'market' becomes 'sale' (I've seen this in other clues).
'grips' is an insertion indicator.
'britain' becomes 'b' (common abbreviation as in GB).
'sale' enclosing 'b' is 'SABLE'.
(Other definitions for sable that I've seen before include "Adventurous courage" , "Bales of precious fur" , "(Fur from) small carnivore" , "Brush" , "Bales of valuable Russian fur" .)