It's green, yes, with a bit of a red back (4)
I believe the answer is:
jade
'it's green' is the definition.
(jade is a kind of green)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for jade that I've seen before include "Wear down" , "Semiprecious stone - exhaust" , "horse not in prime condition" , "Ornamental semiprecious stone" , "Mount" .)