Brownie ingredient (6)
I believe the answer is:
walnut
'brownie ingredient' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for walnut that I've seen before include "Hard dark-brown wood used furniture" , "Popular wood for furniture" , "Tree yielding good wood for furniture" , "A hardwood" , "Valuable furniture wood" .)