Green stone drain (4)
I believe the answer is:
jade
This clue is a double definition.
'green stone' is the first definition.
(I've seen this before)
'drain' is the second definition.
(thesaurus)
(Other definitions for jade that I've seen before include "Worn-out horse" , "disreputable woman" , "Semiprecious stone - exhaust" , "Semiprecious gemstone - old, overworked horse" , "Green stone; hack" .)