Animal doctor collapsing near warhorse (7)
I believe the answer is:
veteran
'warhorse' is the definition.
(I know that warhorse can be written as veteran)
'animal doctor collapsing near' is the wordplay.
'animal doctor' becomes 'vet' (veterinarian).
'collapsing' indicates anagramming the letters.
'near' anagrammed gives 'eran'.
'vet'+'eran'='VETERAN'
(Other definitions for veteran that I've seen before include "Old-stager" , "Retired soldier" , "Nervate (anag.)" , "Warhorse" , "Old soldier's" .)