Pester the old horse (3)
I believe the answer is:
nag
'pester the old horse' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nag that I've seen before include "Persistently worry; horse" , "Worry; old horse" , "Remind constantly" , "Badger with constant complaints" , "Scold persistently, annoy" .)