Experienced old animal doctor? (3)
I believe the answer is:
vet
I believe this clue is a triple definition!
'experienced' is the first definition.
(I've seen this before)
'old' is the second definition.
(thesaurus)
'animal doctor?' is the third definition.
(veterinarian)
(Other definitions for vet that I've seen before include "'Examine this doctor, in short (3)'" , "Dog doc?" , "Examine, check" , "Animal doctor, in brief" , "Ex-serviceman" .)