His patients don't talk (3)
I believe the answer is:
vet
'his patients don't talk' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is all the clue.
Can you help me to learn more?
(Other definitions for vet that I've seen before include "Experienced" , "animal doctor to > investigate" , "Ex-serviceman" , "'Examine this doctor, in short (3)'" , "solider once" .)