How to address a motherly horse? (5)

I believe the answer is:

madam

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'horse?' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

I can't explain the remainder of the clue.

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(Other definitions for madam that I've seen before include "Formal address to a woman - precocious little girl" , "Polite form of address for a woman" , "'Call Me ---' (I. Berlin)" , "pompous young lady?" , "Leader of House of Congress" .)

I've seen this clue in the Evening Standard.

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