Horse enthusiast pursues box (8)
I believe the answer is:
chestnut
'horse' is the definition.
(I know that chestnut is a type of horse)
'enthusiast pursues box' is the wordplay.
'enthusiast' becomes 'nut' (nut can mean an enthusiast or fan).
'pursues' is a charade indicator (letters next to each other).
'box' becomes 'chest' (chest is a kind of box).
'nut' after 'chest' is 'CHESTNUT'.
(Other definitions for chestnut that I've seen before include "Deep reddish-brown" , "Tree - reddish-brown" , "not a cracker!" , "Sweet -; horse -; water -" , "Item for stuffing" .)