Worn-out horse, green in colour
I believe the answer is:
jade
I believe this clue is a triple definition!
'worn-out horse' is the first definition.
(I've seen this in another clue)
'green' is the second definition.
(jade green is a kind of green)
'in' acts as a link.
'colour' is the third definition.
(I know that jade is a type of viridity)
(Other definitions for jade that I've seen before include "Semiprecious gemstone - old, overworked horse" , "Semi-precious gemstone -- tired old horse" , "Worthless horse" , "Wear down" , "Semiprecious stone - exhaust" .)