Box with fruit for horse (8)
I believe the answer is:
chestnut
'for horse' is the definition.
(chestnut is a kind of horse)
'box with fruit' is the wordplay.
'box' becomes 'chest' (chest is a kind of box).
'with' means one lot of letters go next to another.
'fruit' becomes 'nut' (nut is a kind of fruit).
'chest'+'nut'='CHESTNUT'
(Other definitions for chestnut that I've seen before include "Conker tree - 11 of 10" , "Dark golden-brown horse" , "Hunt sect behind a big tree" , "Tree - reddish-brown" , "Sweet -; horse -; water -" .)