Worn-out horse; chop (4)
I believe the answer is:
hack
I believe this clue is a double definition.
'worn-out horse' is the first definition.
'chop' is the second definition.
(I know that chop can be written as hack)
(Other definitions for hack that I've seen before include "Drudge, inferior writer" , "Chop roughly - mediocre journalist" , "Writer of uninspiring stuff" , "Kick on the shins" , "try to access unauthorised data" .)