Horse head on box (8)
I believe the answer is:
chestnut
'horse' is the definition.
(chestnut is a kind of horse)
'head on box' is the wordplay.
'head' becomes 'nut' (informal term for somebody's head).
'on' means one lot of letters go next to another.
'box' becomes 'chest' (chest is a kind of box**).
'nut' after 'chest' is 'CHESTNUT'.
(Other definitions for chestnut that I've seen before include "Wooden" , "Tree (horse or sweet)" , "Item for stuffing" , "''Underneath the spreading . . . . . . . . tree''" , "funny old thing?" .)