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