Complain constantly of a horse (3)
I believe the answer is:
nag
This clue is a double definition.
'complain constantly' is the first definition.
(I've seen this in another clue)
'a horse' is the second definition.
(term for an old horse)
'of' acts as a link.
(Other definitions for nag that I've seen before include "Complain, worry" , "(Of pain) ache dully; horse (colloq.)" , "Badger continually like old horse" , "Old or over-worked horse" , "Mount" .)