Many a retired racehorse (4)
I believe the answer is:
sire
'many a retired racehorse' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sire that I've seen before include "Old form of address to king - rise to it" , "Ancestor" , "Male parent of a horse, for example" , "Stallion; beget" , "Male parent of an animal" .)