They're important to one's standing as a rider (8)
I believe the answer is:
stirrups
'as a rider' is the definition.
I can't judge whether this defines the answer.
'they're important to one's standing' is the wordplay.
I cannot quite understand how this works, but
'standing' could be 'up' and 'up' is found within the answer.
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for stirrups that I've seen before include "Metal loops for rider's feet" , "Riding equipment" , "Horse riders put their feet in them" , "footholds" , "Metal loops for a rider" .)