Worn-out horse endlessly tired (4)
I believe the answer is:
jade
'worn-out horse' is the definition.
(I've seen this before)
'endlessly tired' is the wordplay.
'endlessly' means to remove the last letter.
'tired' becomes 'jaded' ('jade' can be a synonym of 'tire').
'jaded' with its final letter taken away is 'JADE'.
(Other definitions for jade that I've seen before include "Green gem" , "Semiprecious stone - exhaust" , "disreputable woman" , "Green stone; hack" , "Green ornamental stone" .)