Harass useless horse (3)
I believe the answer is:
nag
This could be a double definition.
'harass' is the first definition.
(I've seen this in another clue)
'useless horse' is the second definition.
'nag' can be an answer for 'horse' (term for an old horse). I'm not sure about the 'useless' bit.
Can you help me to learn more?
(Other definitions for nag that I've seen before include "Worn out horse" , "shrew or > badger" , "Complain continuously like old horse" , "Worry; old horse" , "Persistently worry; horse" .)