Brown’s old joke (8)
I believe the answer is:
chestnut
I believe this clue is a double definition.
'brown's' is the first definition.
(shade of brown)
'old joke' is the second definition.
(as in an old chestnut)
(Other definitions for chestnut that I've seen before include "Tree fruit known informally as a conker" , "Laughably predictable" , "Horse coat colour" , "Item for stuffing" , "comedian's overused line?" .)