Incitement for a horse (4)
I believe the answer is:
spur
'incitement' is the definition.
(spur is a kind of incitement)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for spur that I've seen before include "Goad - branch line" , "Impel" , "Cowboy's goading instrument" , "Goad as for a horse" , "Egg on or goad as horseman might" .)