Biscuit: a cookie originally in dark colour (8)
I believe the answer is:
macaroon
'biscuit' is the definition.
(I know that macaroon is a type of biscuit)
'a cookie originally in dark colour' is the wordplay.
'originally' indicates taking the first letters.
'in' is an insertion indicator.
'dark colour' becomes 'maroon' (maroon is a dark red colour).
The first letter of 'cookie' is 'c'.
'a'+'c'='ac'
'ac' put within 'maroon' is 'MACAROON'.
(Other definitions for macaroon that I've seen before include "cake" , "Almond or coconut biscuit" , "Biscuit of egg-white and almonds or coconut" , "Light biscuit" , "Sweet bit of food" .)