Light green — old horse (4)
I believe the answer is:
jade
This clue is a double definition.
'light green' is the first definition.
(I've seen this in another clue)
'old horse' is the second definition.
(I've seen this before)
(Other definitions for jade that I've seen before include "Semiprecious stone - shade of green" , "Mount" , "Semiprecious stone - exhaust" , "disreputable woman" , "Green gem" .)