One of the equines — some nicker (4)

I believe the answer is:

pony

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'one' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for pony that I've seen before include "Small breed of horse" , "Small horse - PS25" , "Animal; PS25" , "staked 25" , "Horse (worth GBP25?)" .)

I've seen this clue in The Times.

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