Knife the horse on film (7)
I believe the answer is:
bayonet
'knife' is the definition.
(bayonet is a kind of knife)
'horse on film' is the wordplay.
'horse' becomes 'bay' (bay is a kind of horse).
'film' becomes 'ET' (E.T. the Extra-Terrestrial).
'bay'+'on'+'et'='BAYONET'
'the' is the link.
(Other definitions for bayonet that I've seen before include "Stabbing weapon" , "Muzzle blade" , "Neat boy (anag.)" , "Lethal Weapon" , "Type of light bulb fitting" .)