Talk the hind leg off a horse? (3)

I believe the answer is:

nag

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'a horse?' is the definition.
(term for an old horse)

I cannot understand how the rest of the clue works.

(Other definitions for nag that I've seen before include "Keep worrying" , "Grumble; horse" , "Be persistently painful" , "trouble constantly" , "Scold like an old horse" .)

I've seen this clue in the Evening Standard.

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