Old or over-worked horse (3)
I believe the answer is:
nag
'old or over-worked horse' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nag that I've seen before include "Harry" , "Persistently worry; horse" , "The horse to annoy continually" , "to get on one's back" , "Scold persistently, annoy" .)