One used by a rider in raising the stirrups? (4)

I believe the answer is:

spur

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'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

I don't understand the rest of the clue.

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(Other definitions for spur that I've seen before include "Rider's heel wheel" , "Drive on" , "Projection on cock's leg" , "'Urge, goad (4)'" , "Goad as for a horse" .)

I've seen this clue in the Evening Standard.

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