Box, security device for horse (8)
I believe the answer is:
chestnut
'horse' is the definition.
(chestnut is a kind of horse)
'box security device' is the wordplay.
'box' becomes 'chest' (chest is a kind of box).
'security device' becomes 'nut' (a nut can be used to secure something).
'chest'+'nut'='CHESTNUT'
'for' is the link.
(Other definitions for chestnut that I've seen before include "Reddish-brown colour" , "Hunt sect behind a big tree" , "Wood" , "Horse coat colour" , "Item for stuffing" .)