Horse box fan (8)
I believe the answer is:
chestnut
'horse' is the definition.
(I know that chestnut is a type of horse)
'box fan' is the wordplay.
'box' becomes 'chest' (chest is a kind of box).
'fan' becomes 'nut' (both can mean an enthusiast).
'chest'+'nut'='CHESTNUT'
(Other definitions for chestnut that I've seen before include "Stale joke or cliche" , "Dark golden-brown horse" , "Old joke - edible kernel" , "Sweet -; horse -; water -" , "Tree fruit known informally as a conker" .)