Menagerie animal? (3)

I believe the answer is:

nag

I'm a little stuck... Click here to teach me more about this clue!

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

'menagerie' is the wordplay.
'NAG' is hidden in amongst the letters.
But, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for nag that I've seen before include "Old Arab maybe" , "Badger with constant complaints" , "Bother persistently" , "Persistently worry; horse" , "Complaint" .)

I've seen this clue in the Evening Standard.

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