Worthless horse (3)
I believe the answer is:
nag
'worthless horse' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nag that I've seen before include "Find fault" , "to get on one's back" , "its head to name the pub" , "Horse; keep scolding" , "Annoy constantly" .)