It's important to one's standing as a rider (7)
I believe the answer is:
stirrup
'a rider' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'it's important to one's standing' is the wordplay.
I cannot really 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 present in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'as' is the link.
Can you help me to learn more?
(Other definitions for stirrup that I've seen before include "Rider's footpiece" , "This helps one to hear" , "Rider's foothold" , "Support for a rider's feet" , "Rider's foot rest" .)