Horse in box, one going to bolt (8)
I believe the answer is:
chestnut
'horse' is the definition.
(chestnut is a kind of horse)
'box one going to bolt' is the wordplay.
'box' becomes 'chest' (chest is a kind of box).
'one going to bolt' becomes 'nut'.
'chest'+'nut'='CHESTNUT'
'in' acts as a link.
(Other definitions for chestnut that I've seen before include "funny old thing?" , "Item for stuffing" , "Old joke - edible kernel" , "Old story" , "Golden to reddish brown colour" .)