About to tuck into brownish-red biscuit (8)
I believe the answer is:
macaroon
'biscuit' is the definition.
(I know that Macaroon is a type of biscuit)
'about to tuck into brownish-red' is the wordplay.
'about' becomes 'ca' (short for circa).
'to tuck into' indicates putting letters inside.
'brownish red' becomes 'maroon' (I've seen this before).
'ca' put inside 'maroon' is 'MACAROON'.
(Other definitions for macaroon that I've seen before include "Almond or coconut biscuit" , "cake" , "Light biscuit" , "Chewy biscuit containing almond paste" , "Biscuit of egg-white and almonds or coconut" .)