Animal doctor near sick warhorse (7)
I believe the answer is:
veteran
'warhorse' is the definition.
('veteran' can be a synonym of 'warhorse')
'animal doctor near sick' is the wordplay.
'animal doctor' becomes 'vet' (I've seen this before).
'sick' indicates an anagram.
'near' with letters rearranged gives 'eran'.
'vet'+'eran'='VETERAN'
(Other definitions for veteran that I've seen before include "One of great experience" , "Old soldier's" , "Old serving man" , "Old-stager" , "Retired soldier" .)