For polo, it's useful to have a certain amount of money (4)

I believe the answer is:

pony

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'amount of money' is the definition.
(slang term for 25 pounds)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

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

I've seen this clue in the Evening Standard.

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