Horse is straddled by female — top rider (10)
I believe the answer is:
showjumper
'rider' is the definition.
(showjumper is a type of horse rider)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for showjumper that I've seen before include "equestrian" , "horse" , "one tackling obstacles in due course" , "Sportsman" , "Sportsperson penalised for a refusal" .)