Thoroughbred's father (4)
I believe the answer is:
sire
'thoroughbred's father' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sire that I've seen before include "Ancestor" , "Male parent of, say, horse" , "Stallion; beget" , "Male parent of a horse, for example" , "Foal's father" .)