Horse training for The Derby consistently bringing up the rear (4)
I believe the answer is:
grey
'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" .)