Horse giving worry (3)
I believe the answer is:
nag
I believe this is a double definition.
'horse' is the first definition.
(term for an old horse)
'worry' is the second definition.
(I know that nag has the is a more specific form of the action worry)
'giving' acts as a link.
(Other definitions for nag that I've seen before include "Henpeck like old horse" , "Old horse or shrewish wife" , "Mount" , "Worry constantly" , "Scold; inferior horse" .)