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Horse training for The Derby consistently bringing up the rear (4)

Ross

I believe the answer is:

grey

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'horse' is the definition.
(I've seen this before)

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

Can you help me to learn more?

(Other definitions for grey that I've seen before include "Colour of ashes" , "Old - colourless" , "showing his age?" , "Dismal - colour" , "Gloomy colour" .)

I've seen this clue in The Telegraph.
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