Person who knows how to treat animals (3)
I believe the answer is:
vet
This might be a double definition.
'person who knows how' is the first definition.
'vet' can be an answer for 'person' (vet is a kind of person). I am not certain of the remainder of the definition.
'treat animals' is the second definition.
'vet' can be an answer for 'treat' (vetting is a kind of treating). I am not sure about the 'animals' bit.
'to' is the link.
Can you help me to learn more?
(Other definitions for vet that I've seen before include "Dog doc?" , "Examine carefully - might one help a lame duck?" , "Check someone for suitability" , "Screen" , "A small animal doctor" .)