Person on horseback (5)
I believe the answer is:
rider
'person on horseback' is the definition.
(someone riding a horse)
This is the entire clue.
(Other definitions for rider that I've seen before include "Equestrian or biker?" , "One in the saddle" , "Supplementary verdict brought in by jury" , "Possibly cavalier" , "Horse(wo)man" .)