Fit to work like a horse? (7)

I believe the answer is:

harness

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

'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 Sun, The Telegraph, The Guardian and The Globe and Mail.

Want a hint initially instead of a full solution? Install my app