Mediocre journalist’s horse (4)
I believe the answer is:
hack
I believe this is a double definition.
'mediocre journalist's' is the first definition.
(I've seen this before)
'horse' is the second definition.
(hack is a kind of horse)
(Other definitions for hack that I've seen before include "Chop; uninspired writer" , "try to access unauthorised data" , "Drudge, inferior writer" , "Journalistic drudge (for the chop?)" , "Journo" .)