12 across
Horse
box,
large
cask
on
back
(8)
Horse box, large cask on back (8)
hints
0
checks
0
hints
The definition is at the beginning of the clue.
How the clue works
Look at Horse — that's your definition, since a chestnut is a type of horse. You get CHEST from box, then take TUN (a large cask) and flip it on back to give NUT, and stack them together to make the full answer.