Tree Arab, say, associated with old joke (5,8)
I believe the answer is:
horse chestnut
'tree' is the definition.
(horse-chestnut is a kind of tree)
'arab say associated with old joke' is the wordplay.
'arab say' becomes 'horse' (arab is a kind of horse).
'associated with' is a charade indicator (letters next to each other).
'old joke' becomes 'chestnut' (I've seen this before).
'horse'+'chestnut'='HORSE-CHESTNUT'
(Other definitions for horse chestnut that I've seen before include "whose shade may be seen in park" , "It bears fruit" , "Tree with conkers" , "Could one be felled like a log" , "Common British tree" .)