Broken-down horse (3)
I believe the answer is:
nag
'broken-down 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 "(Of pain) ache dully; horse (colloq.)" , "Old or over-worked horse" , "Keep criticising, hurting" , "Worry; old horse" , "Remind constantly" .)