About to tuck into reddish-brown biscuit (8)
I believe the answer is:
macaroon
'biscuit' is the definition.
(macaroon is a kind of biscuit)
'about to tuck into reddish-brown' is the wordplay.
'about to' becomes 'ca' (I've seen this in other clues).
'tuck into' means one lot of letters goes inside another.
'reddish-brown' becomes 'maroon' (I've seen this before).
'ca' placed into 'maroon' is 'MACAROON'.
(Other definitions for macaroon that I've seen before include "Biscuit made with almond paste" , "Sweet bit of food" , "Type of biscuit" , "Light biscuit" , "Biscuit of egg-white and almonds or coconut" .)