Cask rolled behind box tree (8)
I believe the answer is:
chestnut
'tree' is the definition.
(chestnut is a kind of tree)
'cask rolled behind box' is the wordplay.
'cask' becomes 'tun' (tun is a kind of cask).
'rolled' is a reversal indicator (letters roll back).
'behind' is a charade indicator (letters next to each other).
'box' becomes 'chest' (chest is a kind of box).
'tun' in reverse letter order is 'nut'.
'nut' put after 'chest' is 'CHESTNUT'.
(Other definitions for chestnut that I've seen before include "Old story" , "Item for stuffing" , "Wood" , "''Underneath the spreading . . . . . . . . tree''" , "Brown horse" .)