Fit to work like a horse? (7)

I believe the answer is:

harness

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'horse' is the definition.
(harness horse is a kind of horse)

I don't understand the rest of the clue.

(Other definitions for harness that I've seen before include "Horse's tackle" , "Control - some tack" , "Exploit (something's) potential energy" , "Fasten (horse to cart)" , "Equine gear" .)

I've seen this clue in The Telegraph, The Sun, The Globe and Mail and The Guardian.

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