Horse box enthusiast (8)
I believe the answer is:
chestnut
'horse' is the definition.
(I know that chestnut is a type of horse)
'box enthusiast' is the wordplay.
'box' becomes 'chest' (chest is a kind of box).
'enthusiast' becomes 'nut' (nut can mean an enthusiast or fan).
'chest'+'nut'='CHESTNUT'
(Other definitions for chestnut that I've seen before include "not a cracker!" , "Tree (horse or sweet)" , "Conker tree - 11 of 10" , "Hunt sect behind a big tree" , "Horse coat colour" .)