Course in horsemanship at Hickstead (4)
I believe the answer is:
path
'course' is the definition.
('path' can be a synonym of 'course')
'in horsemanship at hickstead' is the wordplay.
'in' indicates the answer is hidden within the clue.
'PATH' can be found hidden inside 'horsemanship at hickstead'.
(Other definitions for path that I've seen before include "Course of action; track" , "Footway or track" , "Established route" , "Trodden track" , "Track, way for walkers" .)