Brown's familiar old joke (8)
I believe the answer is:
chestnut
This clue is a double definition.
'brown's' is the first definition.
(shade of brown)
'familiar old joke' is the second definition.
(an old chestnut is an old joke)
(Other definitions for chestnut that I've seen before include "not a cracker!" , "Dark golden-brown horse" , "Tree (horse or sweet)" , "''Underneath the spreading . . . . . . . . tree''" , "Sweet -; horse -; water -" .)