Animal doctor worked near warhorse (7)
I believe the answer is:
veteran
'warhorse' is the definition.
(synonyms)
'animal doctor worked near' is the wordplay.
'animal doctor' becomes 'vet' (veterinarian).
'worked' indicates anagramming the letters.
'near' is an anagram of 'eran'.
'vet'+'eran'='VETERAN'
(Other definitions for veteran that I've seen before include "Retired soldier" , "American ex-soldier or old hand" , "Nervate (anag.)" , "Long-serving" , "Describing a pre-1905 car (or pre-1919 at a pinch?)" .)