Old horse base now divides work (8)

I believe the answer is:

eohippus

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'old horse' is the definition.
'eohippus' can be an answer for 'horse' (eohippus is a kind of horse). I'm unsure of the 'old' bit.

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

Can you help me to learn more?

I've seen this clue in The Telegraph.

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