Old joke (8)
I believe the answer is:
chestnut
'old joke' is the definition.
(as in an old chestnut)
This is all the clue.
(Other definitions for chestnut that I've seen before include "Dark golden-brown horse" , "Sweet -; horse -; water -" , "not a cracker!" , "funny old thing?" , "Stale joke or cliche" .)