Alight from horse (8)
I believe the answer is:
dismount
'alight from horse' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dismount that I've seen before include "Get off a horse/bicycle" , "Alight from a horse or bicycle" , "Cease to ride" , "Get off one's horse" , "Get down, from bike or horse" .)